Application Note

LEM Transducers
Generic Mounting Rules

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9

1. Introduction
The aim of this document is to explain which rules thecustomers have to respect when mounting LEM transducers
in their applications. Respecting the rules described below
will guarantee a good mounting of the product in its
environment and a good behavior in time.

List of figures

Fig. 1: Transducer mounted on the primary
bar OR using housing brackets		1
Fig. 2: Transducer mounted horizontally
OR vertically				2Fig. 3: First contact zone between LEM
transducer and customer’s support badly
located				2Fig. 4: Badly designed customer’s support
effect on LEM transducer’s bracket		2Fig. 5: Local insert effect on customer’s
support around screw head			3
Fig. 6: Insufficient contact on customer’s
support around screw head			3
Fig.7: Examples of non flat customer
support design around screw zone		3
Fig. 8: Screw axis not perpendicular tocontact surface on customer’s support		4
Fig. 9: Washer types allowed for directcontact with LEM transducer’s brackets 4
Fig. 10: Large head screw example
integrating washer function			5
Fig. 11: Potential risk due to the use of
double washer mounting			5
Fig. 12: Loosening torque after 500 h at
23°C with self-locking system or elastic
washer				6This document only concerns mechanical mounting of
the transducers. For example, rules concerning electricalconnections, electromagnetic environment… etc are notdescribed here.

2. Mechanical theory and mounting method choice
When a solid object has to be mounted perfectly (no possible
motion), you only have to constrain six possible movement modes,
called DOF (“degrees of freedom”). For example, defining one point
position (three translations) and orientation (three angles) enables
to know exactly the position of an object in space.
Theoretically speaking, when less than six DOFs are blocked, thesystem is hypostatic and some possible motion modes remain. When
more than six DOFs are constrained, the system is hyperstatic, thatis to say, more constraints than necessary to mount the part areused and there is a risk of over-stressing it.
In order to avoid hyperstatic problems, it’s recommended to mount
LEM transducers with only one mounting system available. Indeed,some transducers can be mounted horizontally or vertically, using
housing brackets or primary bar mounting holes, but in each caseboth mounting system shall not be used together as shown below.
Primary bar fixing
points are free here

Only primary bar fixing
points are used here

Fig. 13 : Torque loss with standard ISO
metric screw+flat plain washer at 23°C.		7
Fig. 14 : Corrosion effect on transducer
secondary connection with badly chosen
nuts.				8Fig. 15 : Metals galvanic compatibility
chart.					9N°ANE120504/1

No contact with customer’s plate

The only GOOD
mounting
possibilities

Only the 4 housing brackets
are fixed on customer’s plate

Fig. 1: Transducer mounted on the primary bar OR using housing brackets

LEM Transducers Generic Mounting Rules

Design rules
for customer’s support

Vertical mounting
through 2 dedicatedholes designed by LEM

Horizontal mounting
through 4 dedicated holes
designed by LEM

OR

Fig. 2: Transducer mounted horizontally OR vertically

3 Design rules for customer´s support
The aim of this section is to give generic rules enabling customer
to design properly the support on which the LEM transducer will bemounted and to avoid some traps.
The main rule is to maximize the volume of plastic directly compressed
while tightening in order to smooth stresses. The higher the stresses,
the more important the risk of having creep phenomenon (loosening
problems of the mounting screws in time).
First of all, the support has to be as flat as possible and the contact
with the transducer has to be located primarily around the mounting
screw zones. Some interferences in other regions before screwtightening would cause damages on mounting brackets as shown
below (mounting bracket bending before compression due to screw
tightening) :Initial contacts
around screw
zones

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9

List of figures

GOOD : First contact zone aroundtransducer mounting brackets

Initial contact
badly located

BAD : First contact badly located with customer’sbase plate (initial gap around mounting brackets)

Fig. 3: First contact zone between LEM transducer and customer´s support badly locatedAnother principle concerning customer’s support design is
to maximize the first contact zone between support and LEM
transducer’s bracket around screw pressure zone. The following
view shows what can happen when using a local insert lower than
general contact surface.

Fig. 2: Transducer mounted horizontally
OR vertically			2Fig. 3: First contact zone between LEM
transducer and customer’s support badly
located			
2
Fig. 4: Badly designed customer’s support
effect on LEM transducer’s bracket
2
Fig. 4: Badly designed customer´s support effect on LEM transducer´s bracket

Page 2

LEM Transducers Generic Mounting Rules

Design rules
for customer’s support

The problem shown above has been seen using a torque value lower
than the one recommended in LEM product datasheet. This can be
explained in the following figures :Fig. 5: Local insert effect on customer´s support around screw head

The latter configuration corresponds to a customer fixing zone builtwith a metal sheet and a welded or a caged nut (see Figure 7). In
both cases, the direct surface of contact between LEM transducer’s
plastic bracket and the nut is lower than general metal sheet surface.
The surface of customer’s support in direct contact with transducer’s
mounting bracket should be at least as wide as the washer used fortightening (see Figure 6). In case of using for example a “Rivkle” pierce
nut (see Figure 7), the following problem could appear :Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9

Fig. 6: Insufficient contact on customer´s support around screw head

List of figures
Caged nut

Fig. 5: Local insert effect on customer’s
support around screw head		3« Rivkle » pierce nut

Fig. 6: Insufficient contact on customer’s
support around screw head		3
Fig. 7: Examples of non flat customer
support design around screw zone3

Weld nut

Fig. 7: Examples of non flat customer support design around screw head

Page 3

LEM Transducers Generic Mounting Rules

Design rules
for customer’s support
Choice of fastening
hardware
The last thing to avoid when designing the customer support is to have ascrew axis not perpendicular to contact surface. In this case, stress will
be very different around the screw hole and the plastic will be damaged.

Fig. 8: Screw axis not perpendicular to contact surface on customer´s support

To conclude, we can emphasize the fact that totally flat customer’s
support design should be used if possible. If not, local nut function
should be implemented very carefully in order to limit non-flat impact
on plastic brackets under screw pressure.

4 Choice of fastening hardware
Choose good fastening hardware will allow mounting LEM transducers
without damaging the plastic brackets, and smoothing as much as
possible the stress around the screw zone in order to limit creep risks.

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9First of all, in order to minimize stress below screw heads, it isimportant to maximize the surface of compressed plastic. So, awasher is mandatory ; standard screw heads dimensions are notwide enough.
Concerning the washer type : its aim is to smooth stress whiletightening, so any aggressive shape in direct contact with plastic
brackets (cutting edges…) should not be used and only flat plain
washers are recommended to tighten transducer’s plastic mounting
brackets (minimum recommended size : according to standard NF
E25-514 «M» series).
The following figures show examples of non appropriate washer
types, not to be used directly on LEM plastic parts.

List of figures
Fig. 8: Screw axis not perpendicular tocontact surface on customer’s support 4
Fig. 9: Washer types allowed for directcontact with LEM transducer’s brackets 4

OK

NOK

NOK

NOK

NOK

Fig. 9: Washer types allowed for direct contact with LEM transducer´s brackets

Page 4

LEM Transducers Generic Mounting Rules

Choice of fastening
hardwareWe can notice that the flat plain washer function can be included byusing a large head screw as shown below. However, it’s better to keepthese two functions separately (screw + flat washer); in the case of one
single part, the friction forces are directly applied on the transducer’s
bracket (torque effect).

Fig. 10: Large head screw example integration washer function

Adding a split or locking washer to the flat plain one is sometime used
to prevent loosening, mainly on steel or cast-iron parts for example. Incase of a very stiff elastic washer, using this technique can be slightly
risky on plastic parts as shown below:

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9Moreover, the locking effect is based on the fact that tensile force,
directly linked to the loosening torque, remains the same during
time. This is true while tightening metal, but creeping is much more
important for plastics. Therefore, even with the additional locking
washer, loosening torque will decrease along time. We do not advise
the use of locking washer as they are not very efficient for plastics(see Figure 11).

List of figures
Fig. 10: Large head screw example
integrating washer function		

5Fig. 11: Potential risk due to the use of
double washer mounting		
5Fig. 11: Potential risk due to the use of double washer mounting

Page 5

LEM Transducers Generic Mounting Rules

Choice of fastening
hardwareAnother way to prevent loosening is to use safety self-locking nuts or
screws. In this case, anti-loosening system consists in adding a highfriction material located in the thread zone. Even if plastic creeps, thenecessary torque needed to loosen the screw remains higher thanwith lock washer solution.
The following graph shows the loss of tightening torque whentightening a typical LEM transducer mounting bracket at two different
initial torques (1 or 3 N.m) :		• With only “standard” flat plain washer,		• With NF flat plain washer and lock washer,		• With NF flat washer + safety self-locking nut.

Fig. 12: Loosening torque after 500h at 23°C with self-locking system or elastic washer

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9

List of figures
Fig. 12: Loosening torque after 500 h at
23°C with self-locking system or elastic
washer			
6

Page 6

As we can see above, “Nilstop” self-locking nut is a better way toprevent loosening on plastic parts than a lock washer. The lock washer
uses plastic reaction below the screw head to prevent loosening butloosening is due to the fact that plastic creeps: its stiffness decreases
in the time.
Adding a lock washer has a very low impact concerning loosening
risks. Self-locking nuts don’t work the same way: it adds a new friction
torque due to the contact with elastomeric coating in the thread zonethat is not directly impacted by bracket-plastic creeping.
Finally, we can state that the wider the contact surface area between
washer and plastic is, the more important maximum torque is. The
simplified one dimension approach even shows that maximum torqueis nearly proportional to this surface area.
So, one way to improve acceptable torque by 50% is to increase thecompressed surface area (surface of contact between plain washer
and the plastic bracket…) by the same percentage.

LEM Transducers Generic Mounting Rules

Screw torque

5 Screw torque
The aim of this section is to explain the recommended fastening torquegiven in LEM transducer’s datasheets.
First of all, we can state that the torque has to be lower than a maximum
value in order to ensure that plastic bracket does not get damaged
after mounting but it also has to respect a minimum value in a waythat the transducer is tightened enough on the customer’s support.
The torque applied on the screw has to respect the specified
recommended value of the torque in transducer’s datasheet. Indeed,
the real value applied on the screw shall not exceed recommended
value in any case. When using power tools with a tolerance of ±20%,
the tolerance on the torque must be taken into account. For example,
if the recommended applied torque is 2.2 N.m (±10%) in the datasheet,the nominal torque of the tool should not exceed the maximum
recommended torque, that is to say 2.42 N.m, divided by 1.2. In thiscase, nominal torque on the power tool has to be lower than 2.0 N.m.
The recommended effective fastening torque should be lower than the
maximum torque. Tightening the plastic part with a specific torqueis not necessary because due to creep it will not prevent loosening
risks. Therefore, being close to the maximum limit is not necessary.
As you can see below, the more tightened the screw is, the more
important torque loss is.

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9

List of figures
Fig. 13: Torque loss with standard ISO
metric screw+flat plain washer at 23°C.		7

Fig. 13: Torque loss with standard ISO metric screw+flat pain washer at 23°C

Page 7

LEM Transducers Generic Mounting Rules

Choice of fastening
hardwareMaterial Compatibility

If we analyze this phenomenon more deeply, the importantcharacteristic will be the maximum pressure that the plastic bracket
can accept locally.
This value depends directly on the thermoplastic used for thetransducer’s housing. In conclusion, the maximum torque given in
the datasheet corresponds to a maximum acceptable pressure below
screw head.
The recommended torque value given in the datasheet corresponds
to the limit value of stress accepted by the plastic when :1) Using the standard metric ISO screw mentioned in the datasheet.
Indeed, there is a tight relationship between applied torque, tensile
force in the screw and the thread pitch (see Kellermann and Klein
formula).
2) Using the standard “normal” ISO flat washer corresponding to thediameter of the used screw.
Using different hardware (non metric thread, other washer type) should
be done by the customer very carefully. In such a case, the maximum
torque value given in the datasheet is not valid anymore.
Then, a method to tighten the transducer on the support would be
to manually tighten the different screws slowly to visually detect thestart of marking in plastic by the washer.
Otherwise, another maximum estimated torque could be estimatedby equivalence using the screw system parameters (one dimensional
simplified approach).

6 Material compatibility
The use of different material for the different elements in a threaded
joint may cause galvanic corrosion problems.

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9

Indeed, each material has its own specific electrode potential. If two
metals or alloys, with very different electrochemical potentials arein contact in an electrolyte, such as moist air can be considered,corrosion can occur as shown in the figure 14 below :List of figures
Fig. 14: Corrosion effect on transducer
secondary connection with badly chosen
nuts.				8Fig. 14: Corrosion effect on transducer secondary connection with badly chosen nuts.

Page 8

LEM Transducers Generic Mounting Rules

Material Compatbility
Ideal LEM configuration

The test corresponding to the corroded parts shown in Figure 14 issalt mist (containing NaCl 5%). This picture has been taken after only
4 days of test. All the fastening elements are in Nickel plated Brass,
except the slotted round nuts which are made of Zinc plated blue Steel.
Quickly analyzing the following chart (see Figure 15), it can be easily
noticed that the potential difference between Nickel and Zinc, the twocoating materials is important (about 950 mV). Indeed, the maximum
acceptable difference commonly used is about 250 mV when salt mist
exposure can occur.

Fig. 15: Metals galvanic compatibility chart.

In conclusion, it is important to notice that all the terminals and different
fastenings mounted on LEM Transducers are made of Nickel plated
Brass.

Contents

1 Introduction				12 Mechanical theory
and mounting method choice		1
3 Design rules for customer’s support		2
4 Choice of fastening hardware		4
5 Screw torque				76 Material compatibility			8
7 Ideal LEM configuration			9

List of figures
Fig. 15: Metals galvanic compatibility
chart.				9The fastenings mounted by the customers on LEM Transducers must
respect galvanic compatibilty with both Nickel and Brass (if Nickel
coating is locally damaged..), in order to avoid corrosion problems.
The best way to be compatible is clearly to use directly Nickel plated
Brass components.

7. Ideal LEM configuration- Perfectly flat massive customer support around screws.
- First contact zones only around the LEM transducer brackets.
- Use of ISO metric threaded screws.
- Use of standard plain flat washers (minimum ISO 7089).
- Respect of the recommended torque (taking into account toleranceson the datasheet recommended torque and on the power tool ifnecessary).
Out of this ideal configuration, tightening torque given by thedatasheet is no longer warranted.

Page 9