Lip counterbalancing mechanism for a dockboard

An adjustable dockboard having an improved lip counterbalancing mechanism. The dockboard includes a ramp that is pivoted at its rear edge to a loading dock and is adapted to be moved from a generally horizontal cross-traffic position to an upwardly inclined position. A lip is hinged to the front edge of the ramp and can be pivoted from a pendant downwardly hanging position to a partially extended position short of its fully extended position by a lip lifting mechanism which is actuated as a consequence of the ramp moving downwardly from the inclined position to the cross traffic position. To aid in swinging the lip to the extended position, a counterbalancing spring is connected through a linkage to the lip, and the linkage provides a relative small counterbalancing force when the lip is in the pendant position, and a substantially greater counterbalancing force when the lip is in the extended position. The counterbalancing mechanism aids in pivoting the lip to the extended position, and also acts to cushion the fall of the lip to the pendant position.

BACKGROUND OF THE INVENTION 
Dockboards of the type shown in U.S. Pat. No. 3,117,332 include a ramp 
having its rear end hinged to a loading dock. The ramp is biased upwardly 
to an inclined position by a spring assembly, and a manually releasable, 
uni-directional hold-down device holds the ramp against elevation but 
permits free downward movement of the ramp. Hinged to the front edge of 
the ramp is an extension lip which is adpated to swing from a downwardly 
hanging, pendant position, to an elevated position in which the lip forms 
an extension to the ramp. The lip, when in the extended position, is 
adapted to engage the bed of a truck or carrier and bridge the gap between 
the ramp and the truck to enable material handling equipment to move 
between the dock and the truck bed. 
As disclosed in U.S. Pat. No. 3,117,332, the lip is automatically pivoted 
to its elevated position by a lip lifting and latching mechanism which is 
actuated by the upward movement of the ramp when the hold-down device is 
released. With the lip extended and the ramp in the upwardly inclined 
position, the ramp is then walked down by the operator until the lip 
engages the bed of the truck. Engagement of the lip with the truck bed 
releases the weight from the lip latching mechanism, thereby releasing the 
latching mechanism so that when the truck pulls away from the dock, the 
lip will automatically return to its pendant position. 
U.S. Pat. No. 3,997,932 discloses a dockboard in which the lip is pivoted 
to its extended position as a consequence of the ramp being walked 
downwardly from its inclined position toward its cross traffic position. 
The lip lifting mechanism includes a toggle unit which is an undercenter 
position when the ramp is in the horizontal cross-traffic position. As the 
ramp is elevated, a latching bar is operatively connected to the toggle 
unit and when the ramp is walked down by the operator from the upwardly 
inclined position, the latching bar moves the toggle unit into an 
over-center position and pivots the lip to the partially extended 
position. The latching bar is then disengaged from the toggle unit. The 
over-center relationship is maintained by the weight of the lip acting 
rearwardly through the toggle unit. 
When the ramp moves downwardly into engagement with the bed of a truck, the 
lip will move to the fully extended position, thereby releasing the weight 
of the lip from the toggle unit and enabling the toggle unit to move to 
its undercenter position by gravity. With the toggle broken, the lip will 
automatically fall to its pendant position when the truck pulls away from 
the dock. 
As the lip has substantial weight, a lip assist or lip counterbalancing 
mechanism has been incorporated with dockboards of the type shown in U.S 
Pat. No. 3,177,332. In U.S. Pat. No. 3,203,002, a lip counterbalancing 
mechanism includes an extension spring, while in U.S. Pat. No. 4,091,488, 
a gas spring is employed to aid in counterbalancing the weight of the lip. 
With the lip counterbalancing mechanism, the greatest counterbalancing 
moment is required when the lip is in the extended position. Consequently, 
the counterbalancing mechanism, as used in the past, had a very small 
counterbalancing moment when the lip was in the pendant position and this 
resulted in a lack of control over the entire range of movement of the 
lip, so that the lip would crash down to the pendant position. Because of 
the noise involved in the falling of the lip, it was necessary to use 
shock absorbers to cushion the lip fall. 
SUMMARY OF THE INVENTION 
The invention is directed to an adjustable dockboard having an improved lip 
counterbalancing mechanism and in particular to a lip counterbalancing 
mechanism to be utilized with a dockboard of the type shown in U.S. Pat. 
No. 3,997,932, in which the lip is elevated as a consequence of the ramp 
moving downwardly from its upwardly inclined position. 
More particularly, the counterbalancing mechanism includes a lip engaging 
arm which is pivotally connected to the lip hinge pin and is engageable 
with the undersurface of the lip. One end of a first link is pivotally 
connected to the arm at a location spaced from the hinge pin axis, and the 
opposite end of the first link is pivotally connected to a second link, 
the opposite end of which is pivotally connected to the ramp. 
Mounted beneath the ramp is an extension spring and the spring is connected 
through a roller chain to the pivot axis between the two links. 
When the lip is in the pendant position, the links are disposed at an acute 
angle with respect to each other and the longitudinal axis of the first 
link is in generaly alignment with the hinge axis, so that the force of 
the spring is acting through a relatively small moment. 
As the lip is moved to the extended position, the links will be moved 
toward an in-line position which is offset from the pivot axis of the lip 
lifting arms and the spring contracts. Thus, even though the spring force 
is reduced as the lip moves toward its extended position, the moment arm, 
through which the spring force is exerted, is increased, to thereby 
provide a progressive increase in the counterbalancing moment as the lip 
is extended. 
At its extended position, the weight of the lip is not fully 
counterbalanced so that the lip will fall by gravity to its pendant 
position when the truck pulls away from the loading dock. As there is a 
reduced, yet considerable counterbalancing force at the pendant position, 
the fall of the lip to its pendant position will be cushioned to thereby 
prevent the lip from slamming down to its pendant position and providing 
smoother and quieter operation. 
Other objects and advantages will appear in the course of the following 
description.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
The drawings illustrate a dockboard 1 which is adapted to be mounted in a 
pit or depression in a loading dock 2. The dockboard 2 includes a frame 3 
or supporting structure and a ramp 4 is hinged at its rear edge to the 
frame. The ramp is adapted to be biased upwardly to an inclined position 
by a spring assembly 5, such as that disclosed in U.S. Pat. No. 3,528,118. 
The spring assembly 5 includes a spring 6 that is connected through a 
lever arm 7 to the rear edge of the ramp and the force of the spring acts 
to move the lever arm 7 forwardly to thereby urge the ramp 4 to an 
upwardly inclined position, as shown in FIG. 1. 
The ramp 4 can be held in any desired position by means of a holddown 
mechanism 8, as disclosed in U.S. Pat. No. 3,646,627. The holddown 
mechanism is a uni-directional device which prevents the upward movement 
of the ramo unless released, while permitting free downward movement of 
the ramp. 
An extension lip 9 is pivoted to the forward edge of the ramp. The lip 9 
and ramp 3 carry aligned hinge tubes 10 which receive a hinge pin 11. With 
this construction, the lip can be pivoted from a downwardly hanging 
pendant position, as shown in FIG. 2 to an extended position, in which the 
lip forms an extension to the upper surface of the ramp, as shown in FIG. 
3. Engagement of the rear edge of the lip 9 with the forward surface of 
the ramp 4 prevents the lip from moving beyond the extended position in 
which it is generally flush with the ramp. 
The lip 9 is moved from the pendant position to the extended position by a 
lip lifting mechanism 12, such as that shown in U.S. Pat. No. 3,997,932, 
and the lip lifting mechanism of that patent is incorporated herein by 
reference. 
In general, the lip lifting mechanism 12 includes a link 13, one end of 
which is pivotally connected to a lug 14 mounted for pivoting movement 
about the axis of the hinge pin 11, and the lug 14 is engageable with the 
undersurface of the lip. The opposite end of link 13 is pivotally 
connected to a link 15 which is connected to a lug 16 that is mounted on 
the undersurface of the ramp. Links 13 and 15 constitute a toggle unit 
which is in the under-center position when the ramp is in the horizontal 
cross-traffic position. As the ramp is elevated, a latching bar 17 
connected to the supporting structure engages the toggle unit, and when 
the ramp is subsequently walked down by the operator from the upwardly 
inclined position toward the cross traffic position, the relative movement 
of the ramp with respect to the latching bar 17 causes the toggle unit to 
move toward an in-line position to thereby move the lip 9 toward its 
partially elevated position. When the ramp has been walked down to a 
predetermined inclination, the toggle unit moves into an over-center 
relationship and the latching bar 17 is disengaged from the toggle unit. 
The over-center relationship of the toggle unit is maintained by the 
weight of the lip acting rearwardly through the lug 14 against the toggle 
unit. 
When the ramp 4 is walked down to a position where the lip engages the bed 
of the truck, the engagement of the lip with the truck bed will move the 
lip to the fully extended position, thereby releasing the weight of the 
lip 9 from the toggle unit and enabling the toggle unit to move by gravity 
to its original under-center position. With the toggle unit broken, the 
lip will automatically fall to its pendant position when the truck pulls 
away from the dock. 
In accordance with the invention, a lip counterbalancing mechanism 18 is 
associated with the lip to aid in counterbalancing the weight of the lip. 
The lip counterbalancing mechanism 18 includes an extension spring 19 
which is located generally horizontally beneath the ramp. One end of 
spring 19 is connected to a lug 20 that depends from the undersurface of 
the ramp, while the opposite end of the spring 19 is connected through an 
adjustable turn buckle 21 to one end of a roller chain 22. Chain 22 passes 
over a sprocket 23, which is journalled within a bracket 24 mounted on the 
underside of the ramp, and the opposite end of the chain is connected 
through a clevis 25 to a bolt or pivot pin 26. 
As best shown in FIG. 4, the ends of a pair of parallel links 27 are 
pivotally connected to the pin 26, and the opposite ends of the links 27 
are pivotally connected to a pin 28 which extends between lugs 29 that 
exend downwardly from the ramp 4. 
The corresponding ends of a pair of links 30 are also pivoted to the pin 
26, while the opposite ends of the links 30 are pivotally connected to the 
end of a lip engaging arm 31 through pin 32. The arm 32 is secured to 
hinge tube 10 which is mounted for rotation about the hinge pin 11. 
As shown in FIG. 3, the arm 32 is provided with a surface 33 which is 
adapted to engage the undersurface of the lip. In addition, the arm 32 is 
provided with a projection or stop 34 which is adapted to engage the rear 
edge of the lip 9 to limit pivotal movement of the arm in a rear or 
clockwise direction, as shown in FIG. 2. 
FIG. 2 illustrates the position of the counterbalancing mechanism when the 
lip is in the pendant position. The links 27 and 30 are disposed in an 
undercenter condition in which they are located at an acute angle with 
respect to each other. The longitudinal axis of the links 30 is in general 
alignment with the hinge pin 11, so that the force of the spring 19, which 
is extended at this time, is acting through a very small moment arm (shown 
by A in FIG. 2) when the lip is in the pendant position. 
As the lip is moved to its extended position, the force of the spring 19 
will move the links 27 and 30 toward an in-line position, as shown in FIG. 
3, so that the force of the spring will be acting through a moment arm B 
to provide a substantially increased counterbalancing moment. Thus, even 
though the spring force itself is decreased as the lip moves from the 
pendant position, the counterbalancing force will be increased due to the 
increase in the moment arm. 
As the truck moves away from the dock, the lip will fall by gravity to its 
pendant position causing the links 27 and 30 to return to their original 
position as shown in FIG. 2. On descent of the lip, the spring is extended 
to provide a cushioning force against the free fall of the lip so that the 
lip will move smoothly and quietly to its pendant position. 
Thus, the invention provides a counterbalancing effect at the lip pendant 
position but yet provides an increased counterbalancing effect as the lip 
is moved toward the extended position. The counterbalancing moment at the 
fully extended position will not fully counterbalance the weight of the 
lip, so that the lip is free to fall by gravity to its pendant position 
when the truck has pulled away from the dock. 
Various modes of carrying out the invention are contemplated as being 
within the scope of the following claims particularly pointing out and 
distinctly claiming the subject matter which is regarded as the scope of 
the invention.