Chapter 1

**Vehicle and Axle-Load Scales**

# [Objectives]{.underline}

After you have studied this chapter, you should be able to:

1\. Describe the basic design of vehicle and axle-load scales and some
special features of the scales.

2\. Describe the basic construction and use of mechanical weighbeam
scales, mechanical dial scales, and electronic digital indicator scales
used in weighing vehicles and axle-loads.

3\. Describe special features of electronic digital indicator scales,
including: on-off switch, zero-setting mechanisms, tare mechanisms, and
push-button print.

4\. Explain the meaning of the following terms:

+-----------------------------------+-----------------------------------+
| > analog type                     | > recording element               |
| >                                 | >                                 |
| > automatic-indicating scale      | > remote indicator                |
| >                                 | >                                 |
| > axle-load scale                 | > scale division, value of (d)    |
| >                                 | >                                 |
| > balance indicator               | > scale section                   |
| >                                 | >                                 |
| > beam scale                      | > steelyard rod                   |
| >                                 | >                                 |
| > digital type                    | > tare                            |
| >                                 | >                                 |
| > graduation                      | > tare mechanism                  |
| >                                 | >                                 |
| > indicating element              | > trig loop                       |
| >                                 | >                                 |
| > indicator                       | > type registering weighbeam      |
| >                                 | >                                 |
| > load cell                       | > unit weight                     |
| >                                 | >                                 |
| > load-receiving element          | > vehicle scale                   |
| >                                 | >                                 |
| > nose-iron                       | > weighbeam                       |
| >                                 | >                                 |
| > pit                             | > weighbridge                     |
| >                                 | >                                 |
| > pivot                           | > weighing element                |
| >                                 | >                                 |
| > poise                           | > zero-load balance               |
| >                                 | >                                 |
| > reading face                    | > zero-setting mechanism          |
| >                                 |                                   |
| > reading-face capacity           |                                   |
+-----------------------------------+-----------------------------------+

> These terms are printed in **bold type** when first introduced.

# [General Considerations]{.underline}

In every commercial transaction involving the weighing of a commodity,
accurate weights and proper weighing practices protect both the buyer
and the seller. The buyer has a right to equity; the seller has a right
to fair value. The livelihoods of those who produce, transport, or
purchase products and materials customarily shipped by truck depend on
accurate measurement of the weights of those goods.

The weights and measures official is responsible for seeing that scales
used to weigh vehicles and their loads, and scales used in enforcement
of laws concerning axle loads, perform properly and that weighing
practices protect the interests of all parties.

# [Applications and Design]{.underline}

**Vehicle scales** are scales adapted to weighing highway, farm, or
other large industrial vehicles (except railroad freight cars), loaded
or unloaded. Sometimes called \"motor truck scales," vehicle scales are
used in buying and selling commodities, and for determining fees for
transporting loads. The sale of farm crops, minerals, timber products,
petroleum, and chemicals can take place based on weights obtained on a
vehicle scale. Charges for transporting goods are also determined with a
vehicle scale. Fees of household movers and motor freight haulers are
set in this way. Some vehicle scales are designed to be movable from one
location to another. Figure 1-1 shows several vehicle scales in use.

![](media/image1.jpeg){width="3.5625in" height="2.0in"}

![](media/image2.jpeg){width="1.823611111111111in"
height="1.6409722222222223in"}

**Figure 1. Vehicle Scales in Use**

An **axle-load scale** is a specific type of vehicle scale. According to
NIST Handbook 44, an axle-load scale is \"a scale permanently installed
in a fixed location, having a load-receiving element specially adapted
to determining the combined load of all wheels (1) on a single axle or
(2) on a tandem axle of a highway vehicle." As can be seen in Figure
1-2, only a portion of the vehicle (in this case, the front wheels of a
truck) actually rests on the scale platform, while the rest of the truck
rests on the pavement surface. Most axle-load scales are permanently
installed in a fixed location. Portable devices, called \"axle-load
weighers" or \"wheel-load weighers" are also used but they are not
covered in this module.

Axle-load scales are used primarily for enforcement of State highway and
traffic laws, and in collecting data for government agencies. In fact,
Handbook 44 requirements apply [only]{.underline} to axle-load scales in
these applications.

  -----------------------------------------------------------------------
  **A.2. Wheel-Load Weighers, Portable Axle-Load Weighers, and Axle-Load
  Scales.** The requirements for wheel-load weighers, portable axle-load
  weighers, and axle-load scales apply only to such scales in official
  use for the enforcement of traffic and highway laws or for the
  collection of statistical information by government agencies.

  **Handbook 44 Scales Code, Paragraph A.2.**
  -----------------------------------------------------------------------

Handbook 44 does not specifically prohibit the use of axle-load scales
in commercial use. However, as you will learn in detail in Chapter 3, it
does require that when vehicles are weighed in commercial applications,
weighing must be performed in a single draft --- that is, that the
entire vehicle must be resting on the load-receiving element at the same
time (unless the vehicle can be separated into parts, each of which can
be weighed as a single draft). \"Split weighing," the practice of
determining the gross weight of a vehicle by adding the weights of
successive drafts of different portions of the vehicle on a single
axle-load scale, does not produce sufficiently reliable accuracy for
commercial use, and so is not permitted.

+-----------------------------------------------------------------------+
| ![](media/image3.jpeg){width="1.823611111111111in"                    |
| height="1.2236111111111112in"}                                        |
+-----------------------------------------------------------------------+
| #                                                                     |
|                                                                       |
| # Figure 1-2. Axle-Load Scale in Use                                  |
+-----------------------------------------------------------------------+

For this reason, a single axle-load scale would not be suitable for
weighing vehicles in commercial applications. Two or more axle-load
scales can be installed in line and used for commercial weighing by
adding the indicated weights, provided that the entire vehicle is
weighed at the same time. However, in such an application, the entire
system, comprising all the individual scales that are used for
single-draft weighing, would be considered to be a vehicle scale.
Special requirements for inspecting and testing multiple scales used in
this way will be described in Chapters 3 and 4.

Their different uses also determine basic differences in size and
capacity between vehicle and axle-load scales:

• The platform of a vehicle scale must be long enough to hold the
longest vehicle to be weighed (typically 5.4 m \[18 ft\], and sometimes
21 m \[70 ft\] or more). In contrast, the platform on an axle-load scale
is relatively short (1.2 m \[4 ft\] to 3 m \[10 ft\]).

• In general, the capacity of vehicle scales is also greater than that
of axle-load scales. Vehicle scales have capacities ranging from 18 to
90 metric tons (20 to 100 tons), while most axle-load scales have
capacities of 27 metric tons (30 tons) or less.

# [Major Components of Vehicle and Axle-Load Scales]{.underline}

Vehicle and axle-load scales have the same basic components:

• l**oad-receiving element**. This is the deck upon which the vehicle
(in the case of an axle-load scale, the portion of the vehicle being
weighed) rests during weighing.

• **weighing elements**. The weighing elements convert the force of the
vehicle\'s weight, which bears upon the load-receiving element, into a
mechanical or electrical signal, which is used to drive the indicating
elements (see below). The weighing elements may be mechanical,
electrical, or a combination of electrical and mechanical.

• **indicating elements**. The indicating elements receive the signal
generated by the weighing elements and produce a visual representation
of the weight. Indicating elements may also be either mechanical or
electrical.

• **recording elements**. While not required by Handbook 44, devices
that make a permanent record of the weight registered are usually found
on vehicle and axle-load scales.

• **approach(es)**. This is the traffic surface, and all its supports,
over which vehicles will normally move when approaching or leaving the
scale.

• **foundation**. The structure or surface upon which the weighing
elements rest.

**Foundation**

In order to provide accurate measurement, the weighing elements must
rest upon a firm, level surface. This is provided by a permanent
foundation of concrete and/or other structural material, installed in
such a manner that it will not be affected by unstable soil conditions
such as those caused by freezing of the ground, heaving of the soil, and
a high water table.

  -----------------------------------------------------------------------
  ![](media/image4.png){width="5.461111111111111in"
  height="3.765277777777778in"}

  **Figure 1-3. Above- and Below-Ground Installation of Scales**
  -----------------------------------------------------------------------

If the scale is designed so that the load-receiving deck is at
approximately ground level, the weighing elements will sit below ground,
in a **scale pit**, as shown in the upper drawing in Figure 1-3.
Alternatively, the scale may be installed so that the deck is above
ground level, with the surface of the foundation at approximately ground
level, as shown in the lower drawing. In either case, access must be
provided to the elements of the scale that are below the deck for
purposes of maintenance and inspection.

#### Approaches

Abrupt or jarring force applied to the weighing elements as a vehicle
moves onto the deck could also affect accuracy. It is, therefore,
essential that the vehicle move smoothly onto the deck, and that any
load the vehicle is carrying be reasonably stable (a shifting load could
also result in abrupt or jarring force on the weighing elements). For
this reason, the portion of the approach(es) adjacent to a vehicle scale
must be smooth, straight, level, and in the same plane as the scale
platform to prevent jarring of the vehicle and its contents.

As can be seen in Figure 1-3, the approaches to a scale often begin with
a slight slope, even if the deck is at approximately ground level. The
reason for this is to facilitate drainage of rainwater away from the
deck. In the case of axle-load scales, a portion of the vehicle actually
rests on the approach during weighing. Since accurate weighing requires
that the load be applied to the weighing elements vertically, it is
essential that the portion of the approach immediately adjacent to, and
level with, the deck be long enough to make it possible for the entire
vehicle to be in the same horizontal plane.

As you will learn in Chapter 3, Handbook 44 includes specific
requirements regarding the construction and dimensions of approaches
relative to the dimensions of the deck for both vehicle and axle-load
scales.

**Load-Receiving Elements**

The deck of a vehicle or axle-load scale must be of a sufficient size in
terms of length and width to accommodate the longest and widest load
that will be applied to it. Since the wheel-width of most trucks is
around 2.4 m (8 ft), decks for both vehicle and axle-load scales are
generally at least 3 m (10 ft) wide. As mentioned earlier, since
axle-load scales measure only the weight on a single or tandem axle,
their decks are usually only about 3 m (10 ft) long. In contrast, a
vehicle scale deck must be long enough to accommodate the longest
vehicle that will be weighed on it, which can be 21 m (70 ft) or more.

  -----------------------------------------------------------------------
  ![](media/image5.png){width="4.958333333333333in"
  height="3.4881944444444444in"}

  **Figure 1-4. Multiple Scale Vehicle Weighing**
  -----------------------------------------------------------------------

In cases where there is a need to know individual axle weights as well
as the total weight of a vehicle, multiple scales may be used for
single-draft weighing. As shown in Figure 1-4, a number of scales, each
with its own load-receiving and weighing elements, are configured in
line. The separate indicators may be joined with a totalizer to show the
individual and combined weights, or a single indicator may be used that
shows the weight on each scale and the total weight.

In this type of installation, even though each individual scale has its
own load-receiving deck, for a given weighing, the
[combined]{.underline} load-receiving element would be considered to
comprise all the separate decks. Note that in the example there is some
**dead space** between successive scale decks; this area is not part of
the scale and cannot be used for weighing (no part of the vehicle could
be resting on this area during weighing). Here, the **load patterns** of
the vehicles may be more critical than the length of the longest
vehicle, since it must be possible to position any vehicle that is to be
weighed so that none of its wheels are resting on dead space. In most
cases, you will find installations where the load-receiving elements are
adjacent to each other without dead space in between.

The scale deck itself is often supported by a structure called the
**weighbridge**, an example of which can be seen in Figure 1-5. Since
the weighbridge bears directly upon the load-bearing points of the
weighing elements (see below), it is generally considered to be a part
of the load-receiving element.

**Weighing Elements**

Figure 1-5 illustrates a typical lever system that is characteristic of
a vehicle or axle-load scale with mechanical weighing elements. The
lever system supports the deck and reduces the force from the load on
the deck by a precise factor (the lever multiple) and transmits that
force to the indicating element. The indicating element (weighbeam or
dial) balances the force and indicates the weight value. It is not
essential for you to study the theory of levers for the purpose of this
module. However, if you want to know more about lever systems, ask your
instructor to review these topics with you.

  -----------------------------------------------------------------------
  ![](media/image6.png){width="4.419444444444444in"
  height="4.8069444444444445in"}

  **Figure 1-5. Typical Mechanical Weighing Elements of a Two-Section
  Vehicle or Axle-Load Scale.**
  -----------------------------------------------------------------------

Note that the weighbridge rests upon four **load-bearing points**. Each
of these points is directly above the load pivot of a **main lever**
(these are 2nd Class levers, with the load point between the fulcrum and
the power point). The power points of each pair of main levers bear on
the load point of an **extension lever**. The extension levers sum the
force of the main levers and transmit it to the **transverse lever**,
which sums the force of the extension levers and transmits the load to
the indicating elements, usually through a vertical **steelyard rod**
(see schematic below the drawing, and Figures 1‑9 and 1-11). The scale
may also be equipped with a **shelf lever**, generally a low-multiple
lever, which receives the load from the platform lever system via the
steelyard rod and transmits its output to the indicating element by
means of a **beam rod** (see schematic).

One additional feature of mechanical weighing elements should be
mentioned, since it provides a means of adjustment that will affect the
registered weight. Most lever systems, like the one shown in Figure 1-5,
are equipped with **nose-irons**. A nose-iron is an assembly that
includes the power pivot of a lever and has a movable mount, so that the
position of the pivot on the power arm can be adjusted by a small
amount. Since this adjustment will lengthen or shorten the power arm of
the lever by a small amount, the lever multiple will also be changed.
One or more of the levers in a mechanical vehicle or axle-load scale may
be equipped with nose-irons. Nose-iron adjustments are made when the
scale is calibrated, and are not performed during normal operation. As
you will learn in a later chapter, specific requirements are included in
Handbook 44 to prevent nose-irons from facilitating fraud.

Electronic weighing elements of vehicle and axle-load scales usually
consist of devices called **strain gauge load cells** (to be
distinguished from pneumatic or hydraulic type load cells). These
operate on the principle that the resistance of a conductor to electric
current varies proportionally with the mechanical stress that is applied
to the conductor: compressive stress (pressing or squeezing) decreases
resistance, tension (stretching) increases resistance. In a strain gauge
load cell, like the one depicted in Figure 1-6, several conductive grids
(the strain gauges) are fixed to a column of metal upon which the force
of the load-receiving element bears directly. The force is transmitted
from the metal column to the strain-gauges. A current of known voltage
is supplied to the strain gauges, so that the output voltage of the
grid, which is transmitted to the indicating element by cable, can be
used as a signal to indicate the force applied to the cell.

  -----------------------------------------------------------------------
  ![](media/image7.png){width="3.9277777777777776in"
  height="2.407638888888889in"}

  **Figure 1-6. Strain Gauge Load Cell.**
  -----------------------------------------------------------------------

The example of fully electronic load cell weighing elements in Figure
1-7 shows the weighbridge on load cells. Signals from the cells are
transmitted to junction boxes or the indicating element, where they are
electronically summed.

Some vehicle and axle-load scales employ a combination of mechanical and
electronic elements. These are commonly referred to as
**electromechanical** scales. A typical electromechanical system might
consist of a platform lever system similar to that shown in Figure 1-5.
However, instead of transmitting the mechanical force of the system to
the indicating elements, the transverse lever would bear upon a strain
gauge load cell, the signal from which would drive an electronic
indicating system.

A **scale section** of a vehicle, axle-load, livestock, or railway track
scale consists of two main load supports, usually transverse (crosswise)
to the direction in which the load is applied. Thus, each pair of levers
in the mechanical scale and each pair of load cells in the electronic
scale constitutes a section. Another way to determine the number of
sections is to count the number of load-bearing points and divide by
two.

+-----------------------------------------------------------------------+
| ![](media/image8.jpeg){width="2.875in" height="1.3854166666666667in"} |
+-----------------------------------------------------------------------+
| **Figure 1-7. Electronic Weighing Elements of a Typical               |
| Multi-Section**                                                       |
|                                                                       |
| **Vehicle or Axle-Load Scale.**                                       |
+-----------------------------------------------------------------------+

Figure 1-8 illustrates the conventional method of numbering load-bearing
points and scale sections: facing the scale from the side on which the
indicator is located, number load-bearing points starting in the far
left corner and proceeding in a clockwise direction; number sections
from left to right. Note that a section includes only the portion of the
deck that is directly over the pair of load-bearing points, plus the
space between them. Most axle-load scales are two-section scales.
Vehicle scales may have two sections or more. The number of sections
needed is generally determined by the length and capacity of the deck.

  -----------------------------------------------------------------------
  ![](media/image9.wmf)

  **Figure 1-8. Diagram of a Two-Section Scale**
  -----------------------------------------------------------------------

During the examination of a vehicle or axle-load scale, you will need to
determine both the number of sections and the **concentrated load
capacity** (**CLC**) of the scale. In the case of vehicle and axle-load
scales, the CLC is the maximum axle-load concentration (for a group of
two axles with a centerline spaced 1.2 m (4 ft) apart and an axle width
of 2.4 m (8 feet) for which the weighbridge is designed as specified by
the manufacturer. The CLC rating is for both test and use. As you will
learn, the nominal capacity of a vehicle, axle-load, or livestock scale
placed in service in 1989 or later is limited by the CLC and the number
of sections. This will be covered in more detail in Chapters 4 and 6.
For scales manufactured prior to 1989, the **section capacity** may be
used as the CLC. The section capacity is the maximum weight that can be
applied to a single section of a scale. While the [scale]{.underline}
may be rated at 45 metric tons (100,000 lb) capacity, an individual
section may only be capable of sustaining a load of 22.6 or 27.2 metric
tons (40,000 or 60,000 lb) directly over a section.

**Indicating Elements**

Three basic types of indicators are used with vehicle and axle-load
scales: weighbeams, dial indicators, and digital electronic indicators.
The first two are used exclusively with scales incorporating mechanical
weighing elements; digital scales can be used either with fully
electronic or electromechanical systems. Thus, vehicle and axle-load
scales may be classified by the design of their weighing and indicating
elements:

• mechanical scales with a full-capacity weighbeam,

• mechanical scales with dials,

• mechanical scales with electronic digital weight indicators
(electromechanical scales), and

• fully-electronic scale systems.

Figure 1-9 shows a typical weighbeam assembly for a vehicle or axle-load
scale, although not all of the elements depicted are required to be on
every weighbeam. You should study this drawing while you read the
following description.

  -----------------------------------------------------------------------
  ![](media/image10.png){width="4.9430555555555555in"
  height="3.290277777777778in"}

  **Figure 1-9. Typical Weighbeam Assembly**
  -----------------------------------------------------------------------

The weighbeam itself is actually both a weighing and an indicating
element. As the final lever in a mechanical scale (that is, the lever
furthest removed from the load-receiving element), the weighbeam is a
weighing element. In addition, the position of the weighbeam indicates
the balance condition of the scale: when the weighbeam is perfectly
horizontal, the load on the scale is balanced, that is, supported
entirely by the weighing elements.

Weighbeams are referred to as **non-automatic indicators**, because the
scale operator must perform actions to obtain the measurement: weight
must be applied to the power arm of the weighbeam until it balances the
load on the deck. The load could be balanced by applying **counterpoise
weights** to the tip of the weighbeam. The weight of the load would then
be the total counterpoise weight times the lever multiple of the system
when the weighbeam is in a balance condition. However, this method could
involve the manipulation of a number of relatively small counterpoise
weights for any given weighing, plus some computation, and would thus be
prone to operator error. In addition, the fact that the counterpoise
weights would have to be readily removable and replaceable from the
hanger could facilitate fraud. A more efficient and secure method
involves balancing the load by moving a sliding **poise** that is fixed
to the weighbeam in a position that balances the applied load. Since the
length of the power arm (and, thus, the scale multiple) varies with the
position of the sliding poise, a single weight can be used to balance a
considerable range of loads. In addition, **graduations** can be
provided on the weighbeam, so that the multiplied weight required to
balance the load can be read directly, without computation.

In fact, although the sliding poise can be used in conjunction with
counterpoise weights, most vehicle and axle-load scales with weighbeam
indicators are **full-capacity weighbeams** (see Figure 1-9). This means
that all loads that are weighed on the scale can be balanced by the
position of the weighbeam poise, without having to add or remove
counterpoise weights. The **counterbalance weights** and **balance
ball** are used to obtain a balance condition when there is no load on
the deck, and are not manipulated during weighing. In fact, on a
full-capacity weighbeam, all weights used to obtain a zero-load balance
should be designed so that they cannot be readily manipulated and so
facilitate fraud. Specific requirements relating to these elements will
be described in detail in Chapter 3. Most full-capacity weighbeam
vehicle scales have a nominal capacity of 100,000 or 120,000 lb.

Now let us consider the weighbeam as an indicating element. As said
earlier, the balance condition of the scale is indicated when the
weighbeam is perfectly horizontal. In fact, when a load is applied to
the deck, the tip of the weighbeam will move upward. When the poise is
moved to a position that balances the load, the weighbeam will
oscillate, or move up and down for some period before coming to rest.
This movement, referred to as the **travel** of the weighbeam, is
limited by the **trig loop**. The trig loop is a loop of metal, oval or
oblong, through which the tip of the weighbeam passes; when the
oscillating beam contacts the top or bottom of the trig loop it comes to
a stop, indicating an out-of-balance condition. When the scale is
balanced, the weighbeam tip should oscillate within the trig loop,
touching neither limit. When the trig loop is properly adjusted, the
oscillating tip of the balanced weighbeam should travel the same
distance from the top and bottom limits before reversing direction.

The balance condition is usually determined by the operator by observing
the travel of the weighbeam tip within the trig loop. However, you may
find some weighbeam scales that are equipped with **balance
indicators**, like the one shown in Figure 1-9. These indicators
generally include a **dashpot** to damp the weighbeam oscillations, and
thus provide a quicker indication of the balance condition than
observing the trig loop. In the example shown, when the arrow coincides
with any point within the black band, the scale is balanced. Another
type of balance indicator includes graduations which are numbered to
represent weight values by which the scale deviates from a balanced
condition. Because of the graduations, this latter type of balance
indicator must be treated as a dial indicator during field examinations.

Figure 1-10 shows a detail of the graduated weighbeam and poise. In
fact, this is a double poise, consisting of a main poise and a
fractional bar. For the moment, consider only the main poise. The
graduations on the weighbeam are the vertical lines extending from the
top of the weighbeam. Each graduation is numbered. Note that the upper
left edge of the poise is shaped to form an edge and pointer. In the
example, the edge and pointer are aligned with the graduation marked
\"31."

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  ![](media/image11.png){width="4.823611111111111in"
  height="3.328472222222222in"}

  **Figure 1-10. Type-Registering Weighbeam Poise**
  -----------------------------------------------------------------------

The top of the weighbeam is notched, and the poise is equipped with a
**pawl**, a spring-loaded tooth that locates the poise and locks it in
place when the pawl is directly over a notch. The handle on top of the
poise operates the pawl: when pressed down, the spring pressure on the
pawl is released, permitting the poise to slide on the weighbeam. When
the handle is released, the spring pressure is applied to the pawl,
which then engages the next notch, locating and locking the poise at
that point. The notches on top of the weighbeam are located so that when
the pawl locates in one, the edge and pointer indicator will align
exactly with a graduation. Each **graduated interval** on this weighbeam
represents 1,000 pounds effectively applied through the lever system to
balance the load. According to the indicator on the main poise, 31,000
lb has been applied.

Next, consider the **fractional bar**, which is really a secondary poise
that is positioned relative to the main poise. Once the main poise has
been positioned, the fractional bar knob is turned until the balance
condition is indicated. Turning the knob moves a weight bar (the
fractional bar) either toward the load pivot or toward the power pivot.
This adjustment affects the power arm, and thus the lever system
multiple in the same way as moving the main poise does, but to a much
smaller degree per unit of distance that the fractional bar is moved.
The graduated intervals on this fractional bar each represent 10 lb; the
reading shown in the example is 400 lb. Since 10 lb is the value of the
smallest graduated interval on this scale, it is also considered the
**scale division** for this scale, designated \"**d**." The scale
division, like the section capacity or CLC, must be determined during
the examination of a vehicle or axle-load scale.

The capacity of the fractional bar (that is, the maximum amount of
effective counterforce that can be applied by adjusting the bar
position) is equal to the value of the graduated intervals on the main
poise, minus one scale division --- 990 lb. Note that the fractional bar
is also graduated and that the cutout in the main poise through which
these graduations are visible also forms a pointer. When the weighbeam
is balanced, the operator adds the indications on the two poises to
obtain the total gross weight of the load to the nearest 10 lb, in this
case, 31,400 lb.

This poise is also equipped to produce a recorded representation of the
weight. We will consider this feature below, under **Indicating
Elements**.

Figure 1-11 shows the major elements of a typical dial indicator that is
used with vehicle and axle-load scales. The force of the load, reduced
by the lever system, is transmitted by the steelyard rod to a shelf
lever, which is enclosed in a cabinet directly below the indicator. The
output of the shelf lever, transmitted by a beam rod, actuates the
mechanical dial mechanism, usually a double pendulum, which acts as a
counterforce. The displacement of the counterforce mechanism by the
applied load drives the dial hand shaft by means of a rack and pinion.
Dashpots are used to reduce oscillation of the indicator, and also to
protect the indicator from mechanical shock when the load is applied. As
with weighbeam scales, a balance ball is provided on the butt of the
shelf lever for adjusting the system to a zero-load balance condition.

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  ![](media/image12.png){width="5.123611111111111in"
  height="3.890277777777778in"}

  **Figure 1-11. Elements of an Automatic-Indicating Dial Scale**
  -----------------------------------------------------------------------

The shelf lever, is functionally similar to a weighbeam. One reason why
a shelf lever is needed on this type of scale is related to the
limitations of the dial mechanism, which is only capable of indicating
over a range that is considerably less than the capacity of a vehicle
scale. Typically, a dial will have a capacity, called the **face
capacity**, of 20,000 lb. In order to utilize the dial for large
capacity scales, counterforce must be applied to the shelf beam when the
load exceeds the face capacity of the dial. This is accomplished by the
application of **unit weights**, which are enclosed within the sealed
cabinet, and are applied or removed from the shelf lever by a mechanism
that is actuated by the scale operator.

Each unit weight, when applied to the shelf lever, provides a
counterforce that is equivalent to the face capacity of the dial. The
scale will have sufficient unit weights to balance the nominal capacity
of the scale, minus the face capacity. When one or more unit weights are
applied, an indication of the total unit weight counterforce is shown in
a window on the dial face, referred to as the **range window**, as shown
in Figure 1-12.

For example, consider a mechanical vehicle scale with a nominal capacity
of 100,000 lb, equipped with a dial indicator with a face capacity of
20,000 lb, and smallest graduated intervals on the dial face
representing 20 lb (similar to that shown in Figure 1-12). A vehicle
with a gross weight that is estimated to be between 50,000 and 60,000 lb
is to be weighed. Because the estimated weight is between two and three
times the face capacity of the dial, two 20,000-lb unit weights would be
applied. The range window would then indicate 40,000 lb. When the
vehicle load is applied to the scale, the operator adds the weight
indicated by the dial to the amount shown in the range window to
determine the total weight.

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  ![](media/image13.png){width="5.190277777777778in"
  height="2.859722222222222in"}

  **Figure 1-12. Dial Face with a Single Range Window**
  -----------------------------------------------------------------------

Some dial scales are equipped with **automatic unit weights**. The scale
senses the amount by which the load exceeds the face capacity of the
dial and automatically applies the appropriate number of unit weights.
Regardless of whether the unit weights are applied by the operator or by
such a mechanism, scales equipped with dial indicators are considered to
be **automatic-indicating scales**, since the indicator itself is
actuated by a mechanism, and not, like the poise on a weighbeam, by the
operator.

Another function of the shelf lever is to provide tare capability.
Again, the operation is similar to that of the weighbeam: one or more
poises on one or more graduated bars fixed to the shelf lever are used
to balance the tare (that is, the unloaded vehicle). With these poises
in place, when the loaded vehicle is positioned on the scale, the
indicated weight will be the net weight.

Finally, notice that the system shown in Figure 1-11 is equipped with a
**counterbalance weight**. A counterbalance is a weight that is applied,
to the butt end of the shelf lever to produce the effect of a load of a
known weight on the platform. The counterbalance weight, like the unit
weights, is sealed within the cabinet, and is applied or removed using
external controls. The counterbalance weight provides the installer with
a means of balancing out the dead weight of the platform. Fine zero
adjustments are made using a balance ball that is inside the cabinet and
is accessed through a hole in the side of the cabinet. A tool such as a
screwdriver is used to make adjustments.

Mechanical vehicle scales have been used for over 100 years, while the
electromechanical and electronic scales have only been in use since the
development of load-cell technology in the 1960s. Most new installations
are electronic, and many mechanical systems have been adapted to
electromechanical scales. One major reason for the popularity of
electronic equipment is that electronic indicating elements provide
greater and more extensive capabilities than mechanical indicators:

• The electrical signal produced by the weighing elements (load cells)
is transmitted to the indicating elements by cable rather than by a
mechanical linkage. This makes it possible to install remote readouts
and to interface the scale with other electronic devices, such as
computers and printers, which may also be efficiently located at remote
stations.

• Electronic indicators have processing and memory capabilities, making
it possible to enter correction and calibration factors from a control
console rather than having to perform mechanical adjustments.

• Two or more electronic scales can be connected to a single indicator,
which can display weight registration for each scale, and can also
display a totalized weight. This capability greatly simplifies single
draft weighing using multiple scales.

• Extensive tare capabilities are available, as will be described below.

In addition, because electronic digital indicators display quantities
and descriptors in numbers and letters, they are generally easier to
read than dials or weighbeams. As you have learned, mechanical
indicators, especially on large-capacity scales like vehicle and
axle-load scales, often require the operator to observe and sum two
indications in order to determine the scale registration, and thus may
be more prone to operator errors in arithmetic and transcription.

Before considering some of the features of electronic indicators, it is
important that you understand the distinction between **analog devices**
and **digital devices**. Most mechanical indicators, like those
described above, are analog devices.

  -----------------------------------------------------------------------
  **Analog** **type.** A system of indication or recording in which
  values are presented as a series of graduations in combination with an
  indicator, or in which the most sensi­tive element of an indicating
  system moves continuously during the operation of the device.

  **Handbook 44 Definition of an Analog Device**
  -----------------------------------------------------------------------

You will recall that both the weighbeam and dial indicator have
graduations that are used with indicators (the edge and pointer on the
weighbeam poise, and the pointer on the end of the dial hand). In both
types of mechanical indicator, larger quantities are indicated in
increments (by the notched weighbeam bar and unit weight range window).
However, as stated in the definition, indication on the most sensitive
elements of both indicators (the fractional bar and dial face scale) are
continuous: the indicators pass through an infinite number of
intermediate indications between graduations, even if it is not
practically possible to read with accuracy to more than one or two
subdivisions of the graduated intervals. For example, the value of the
graduated interval on the weighbeam fractional bar illustrated in Figure
1-10 is 10 lb. It will often be the case, however, that the beam will be
balanced when the pointer is between graduations. Depending upon the
sensitivity of the weighbeam, it may be possible to accurately read the
indication to the nearest 5 lb. The value represented by the smallest
graduated interval on an analog device (10 lb in this example) is the
**scale division**, conventionally designated \"**d**."

In contrast, digital indicating devices display quantities in numbers,
and indicate values incrementally, the increment being the scale
division, \"d."

  -----------------------------------------------------------------------
  **Digital type.** A system of indication or recording of the selector
  type or one that advances intermittently in which all values are
  presented digitally, or in numbers. In a digital indicating or
  recording element, or in digital repre­sentation, there are no
  graduations.

  **Handbook 44 Definition of a Digital Device**
  -----------------------------------------------------------------------

Most vehicle and axle-load digital indicators have scale divisions of 10
kg (20 lb), and so are not capable of indicating except in multiples of
10 kg (20 lb.) The electronic system is designed to \"round" values to
the nearest increment. For example, if a digital indicator registers a
load of 15, 000 kg (30,000 lb) and a 5 kg (15 lb) weight is added to the
load, the indicator will register 15,010 kg (30,020 lb), rounding the
value to the nearest value that it is capable of displaying.

Most strain gauge load cells produce an analog signal (the output
voltage varying with the applied load). This signal must be converted to
a digital signal by an electronic device that is part of the indicating
element (an **analog-to-digital** or **A/D converter**). The digital
signal is then converted to a displayed weight value by a
microprocessor. It is during this conversion that the \"rounding"
occurs. Some designs of load cells are also completely digital.

It might seem from this description that analog indicators are more
accurate than digital indicators. This is not the case: if the scale
division is appropriate to the application, both types are equally
accurate. However, the difference between the two devices can be
significant from the point of view of test procedures. In cases where
tolerances are not exact multiples of the scale division, the \"zone of
uncertainty" created by the rounding feature of a digital indicator must
be taken into account. This factor will be discussed in greater detail
in Chapter 6.

Figure 1-13 illustrates a typical electronic digital indicator console.
If the scale is installed outdoors, this type of unit would probably be
located in an area where it could be sheltered from the weather, such as
the operator\'s shed. An auxiliary indicator --- usually a \'scoreboard'
display --- would be provided in this case near the scale platform,
where it could be seen by the driver during the weighing.

The display window across the upper portion of the console will have a
liquid crystal or light-emitting diode (LED) number and label display.
The label shown to the right (here, LB GROSS) would automatically change
if the operator selected another weight parameter (e.g., TARE). The
system might also be programmed to display prompts for the operator in
this window.

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  ![](media/image14.png){width="2.1506944444444445in"
  height="1.898611111111111in"}

  **Figure 1-13. Electronic Digital Indicator**
  -----------------------------------------------------------------------

The numeric keypad below the display window is used by the operator to
enter values or codes that are used to select display modes or features.
The remaining buttons on the console are used to activate features, many
of which are available only on electronic indicating devices. The most
common of these features (and the ones with which you will be most
concerned during field examinations of vehicle and axle-weight scales)
are:

• **On-Off Switch**. This switch controls the power supply. Sometimes it
will also perform the system segment check that tests all the lights in
the digital display and verifies whether each is working. Some on-off
switches are designed to shut off power only to the display, thereby
keeping the electronic circuitry warmed up.

• **Zero-Setting Mechanism**. This device makes adjustments to maintain
the scale in a zero-load balance condition. The semi-automatic
zero-setting mechanism requires a single action by the operator. An
example is the **push-button zero**, common on electronic scales. The
scale may also have an auxiliary \"center of zero" indicator that shows
when zero-load balance has been achieved to within one-fourth scale
division or less. Push-button zero mechanisms should [not]{.underline}
be used to set tare: since the tare function would not be indicated on
the display, this practice could facilitate fraud.

• **Tare Mechanism**. A tare mechanism balances out material on the
scale deck not to be included in the net weight. For vehicle scales,
this means the tare mechanism serves to subtract the weight of the
vehicle itself from the total or gross weight. Two types of tare
mechanism are common with vehicle scales having electronic digital
weight indicators: **push-button** and **keyboard tare**. You may also
find some scales with **thumbwheel** or **dial tare**.

> With **push-button tare**, the empty vehicle is placed on the scale,
> and the indicator shows its weight. Then the operator presses and
> releases the tare button and the indicator shows a weight of \"zero,"
> and indicates that the display is \"net weight." When the same truck,
> now loaded, is on the scale, the indicator will show only the weight
> indication for the load: the net weight. In some systems, the gross or
> tare weight may be stored in memory, along with a tag that associates
> the weight with a specific vehicle; the operator can retrieve the
> entered weight when the vehicle returns to the scale to determine the
> net weight. This makes it possible to weigh other vehicles while one
> that has been weighed is being loaded or unloaded.
>
> With **keyboard tare**, first the weight of the tare material (empty
> vehicle) is determined. This value is entered on a keyboard pad, using
> the digits 0-9, and the tare key on the pad is pressed. On some
> systems, the tare value then shows in the weight display for
> verification. On other keyboard tare systems, the tare value is
> displayed on a separate tare display.
>
> A **thumbwheel** (or dial) tare mechanism also requires that the
> weight of the tare material be determined separately. Then thumbwheels
> are turned until each digit in the tare weight is correctly shown in
> the window or next to the indicator line of the thumbwheels. Once the
> numbers have been set, this weight value will automatically be
> subtracted from the gross weight on the deck.
>
> These tare-taking capabilities are convenient for the operator, and
> minimize the risk of operator error (especially the semi-automatic
> push-button tare). However, they can be abused. Whatever the tare
> mechanism, every digital scale must have some means to indicate when
> tare has been taken.

• **Push-button Print**. This device activates a printer that can record
weight values.

##### Recording Elements

Although not required by Handbook 44, devices that make a permanent,
printed record of the transaction are found on most vehicle and
axle-load scales. A wide range of printer designs is available.

The weighbeam depicted in Figure 1-9 is known as a **type-registering
weighbeam** because the registered weight can be impressed onto a ticket
by means of type that is cast onto surfaces of the main beam and
fractional bar. On the detail drawing of the poise shown in Figure 1-10,
note that there are two slots in the side of the poise, one marked
\'TARE,' the other \'GROSS.' When a ticket is inserted into one of the
slots and the handles beneath the poise squeezed together, an impression
is made on the ticket of the weight corresponding to the current
position of the poise and fractional bar.

The tare and gross ticket slots are offset so that both tare and gross
weights can be impressed on the same ticket. An empty vehicle may be
weighed and the registered weight impressed on a ticket placed in the
TARE slot; the device will also impress the word 'tare' next to the
quantity. When that vehicle is loaded and weighed again, the same ticket
is inserted in the GROSS slot and another impression made; the second
impression will be made above the first, and automatically labeled as
gross weight. The operator must then manually subtract the tare from the
gross weight to determine the net weight. Note that the impressed weight
will be in numbers that are multiples of the scale division. The
type-registering printer is thus a digital device use in conjunction
with analog weighing and indicating elements. As you will learn,
Handbook 44 requires that all recorded representations be digital.

Ticket printers can also be used in conjunction with mechanical scales
with dial indicators.

Digital printers are similar in design and operation to the dot-matrix
and inkjet printers that are used with computers. They are actuated by
the same signal that produces the digital visual indication, and the two
should thus agree exactly. If tare/gross weight is stored in system
memory, the ticket can be printed in a single operation.

# [Summary]{.underline}

Axle-load scales are used to determine the weight of single and tandem
axles of highway vehicles in connection with the enforcement of traffic
and highway laws. Vehicle scales are used to determine the weight of
many different types of vehicle loads where sale price of goods or
materials is determined by weight, or where the fee for transfer of
goods is determined by their weight. Vehicle scales are generally larger
and have greater weighing capacity than axle-load scales.

Some vehicle and axle-load scales work on the principle of the lever and
are referred to as mechanical scales. These scales have various kinds of
indicators: the most common are weighbeams and dials. Scales using
strain gauge load cells as the weightsensing element have electronic
digital indicators. Scales that use only load cells are fully
electronic. However, mechanical scales can be adapted for use with
electronic digital indicators by having an element of the lever system
bear upon a strain gauge load cell; these are known as electromechanical
scales.

The function and operation of special features frequently found on
electronic vehicle and axle-load scales must be understood before
examining these devices in the field. These features include: on-off
switch, zero-setting mechanisms, tare mechanisms, and push-button print.