Development and principles
The evolution in motor car bodywork design and
construction has called for many changed methods
of handling repairs. This in turn has demanded an
increasing amount of repair equipment to augment
the traditional hand tools and equipment of the
body builder, panel beater and sheet metal worker.
Modern equipment has been made necessary by the
chassisless construction of modern mass produced
Craft techniques and minor accident damage 373
Figure 13.49Snap 4 tonne standard set supplied for carrying case (Blackhawk Automobile Ltd )
1 Manual pump with hose
2 4-T Bantam hydraulic hose, 18 m
3 4-T ram toe
4 Extension tubes
5 Snap flex head
6 90° V base
7 Tube connector
8 Serrated saddle
9 Wedge head
10 Plunger toe
11 Spread ram
12 Flat base
once the inner construction has been restored to
its normal position, the outer one will have been
corrected at the same time. Before any correction
can be made of such damage it is necessary to
restore this inner construction. This is generally
done by applying pressure to the damaged member
or members. Where sharp kinks or creases have
been formed at any point in the inner construction,
it may be advisable to use heat while the pressure
is being applied, but only on low-carbon steel and
not on high-strength steel. This permits the metal
to return to its original shape with little danger of
cracking.
In using the body jack, it is important to understand
that pressure is being applied at both ends of
the jack simultaneously; therefore there could be
a danger of distorting adjacent undamaged panel
assemblies during the jacking process if the pushing
points are not carefully selected. Such pressure
can if necessary be applied either locally or spread
over larger areas by the introduction and use of
pressure pads, which are usually hard wood blocks.
The body jack is also useful for providing support
or pressure at otherwise inaccessible portions of
the outer panels, as well as applying controlled
pressure in a higher degree than is possible with
hand tools on the various panels.
Basic equipment
A body jack consists of three basic units
(Figure 13.51): a pump, a flexible hose connecting
pump to ram, and the ram unit. The pump comprises
reservoir, pump handle and hose, and is
controlled by a simple open and close release
valve. The handle can be screwed into the pump in
two different positions for ease of operation. The
hose is connected to the ram by a simple quickrelease
coupler which needs to be only finger tight.
Pressure is applied by closing the pump release
valve and operating the pump handle. The pump
will build up sufficient pressure only to overcome
the external resistance against the ram. The need
to apply excessive pressure to the pump handle
indicates that the ram has reached the limit of its
movement. The ram is designed with a snap-on
374Repair of Vehicle Bodies
correct sequence. The corrective force should be
applied as near as possible in the direction opposite
to the force which caused the damage. The body
jack set-up should be applied so as not to push at
the deepest point on the damaged section; instead
work round the outer edges in ever-decreasing
circles which will tend to spring the remaining
damage into the final position. If the body jack
set-up is applied directly against the lowest part
of a damaged section without relieving the strain,
then as the pressure is applied the metal surface
may become kinked and stretched and require
further attention to return it to its correct level.
19 18 17 16
20 21 22 23 24
12 13
Figure 13.50Body jack kit on a mobile trolley (Blackhawk Automobile Ltd )
1 Slip lock extension
2 Pump
3 10 tonne ram
4 10 tonne short ram
5 Wide-angle wedge head
6 Pull plates
7 Pull ram
8 Midget ram
9 Pull rings
10 Lock-on extension tubes
11 Male connector
12 Lock-on tube connector
13 Lock-on female and male connectors
14 Pull clamp
15 Wedgie ram
16 Flat base
17 Serrated saddle
18 Spreader plunger toe
19 Clamp toe lock pins
20 Pull chain
21 90° ‘V’ base
22 Wedge head
23 125 mm flex head
24 Trolley and press stand
system to enable extension tubes and attachments
to be positioned to harness the hydraulic power for
any desired type of application.
All collision damage repair work which makes
use of the body jack equipment is carried out by
using one or another of a number of simple set-ups
or, in the case of more complicated repair, a
combination of set-ups (Figure 13.52). The first
important step therefore is to understand the setups
that can be built with this equipment, the
attachments required and their application; then it
is a question of breaking down a job into its basic
set-ups and applying the corrective force in the
Craft techniques and minor accident damage 375
13.14 Application of the body jack
Pushing
Pushing (Figure 13.53) is the simplest operation of
all and is achieved by inserting the ram between
two points and operating the pump. The plunger
extends until it touches the point at which the load
is to be applied, and as pumping is continued
pressure is built up to overcome the resistance of
the metal at the point of application. Movement of
the damaged area will take place as long as pumping
is continued.
Care must be taken when selecting a ram
anchor; for example, if a ram was placed between
two chassis members and it was intended to push
from the undamaged member to straighten the
damaged member, the force applied would not
rectify the damage but would distort the undamaged
member! The first essential is to ensure that
the pushing anchor point is stronger than the
point which is receiving the corrective force. This
can be done by attaching a base plate to the bottom
Figure 13.52Body jack combinations (Blackhawk Automotive Ltd )
Figure 13.51Hydraulic body jack (Blackhawk
Automotive Ltd )
376Repair of Vehicle Bodies
end of the ram to spread or distribute the load,
which can be spread over an even larger area by
putting a piece of solid hardwood timber between
the base plate and the pushing point. It is seldom
that a pushing application can be achieved using
the ram only, because of its limited travel; therefore
there are available various combinations of
extension tubes, couplings and pusher heads to
cater for any repair requiring straightforward push.
Pulling
Pulling (Figure 13.54) is also a simple operation
but uses a slightly different type of ram. With the
standard equipment a pull converter set is available
to enable the one push ram to fulfil both functions,
but it gives an off-centre pull and is less convenient
to use than the separate pull ram. With the direct
set-ups using the pull rams, the tubes are under
tension and there is no risk of bending. The thread
sections of tubes and couplings are now under
tension and it is the threads themselves that carry
the load, so it is essential to ensure that they are
kept clean, free from dirt and damage and are
securely mated. The pulling combination obtainable
makes use of the pull rams and direct pulling
attachments, though it is also possible to use chain
plates and chains for obtaining a pull with a push
ram. The latter method is quite convenient in such
cases as pulling across the width of a body from
an undamaged door pillar which has been bowed
outwards. It would be necessary to reinforce the
undamaged pillar with timber to prevent distortion
and to protect both pillars from marking by the
chains with suitable packing material.
Spreading
Spreading (Figure 13.55) is a similar application to
pushing except that in the latter case there is sufficient
room for access for the ram and extension
between the two members to be moved apart to
Figure 13.53Pushing with body jack (Blackhawk Automotive Ltd )
Figure 13.54Pulling with body jack (Blackhawk Automotive Ltd )
Craft techniques and minor accident damage 377
permit a direct push. When this is not possible a
means must be found of inserting jaws or attachments
which are capable of applying an indirect
thrust. The most obvious means of spreading is
provided by the wedgies and spread rams. This is
an off-centre load once more, and even under the
most favourable conditions it is not possible to
apply a force of more than about 7 tonnes with the
10 tonne ram, or 2.5 tonnes with the 4 tonne ram.
Stretching or tensioning
The technique of stretching or tensioning
(Figure 13.56) is another means of obtaining a pull
using a push ram. It is different from the type of
pulling previously described, which was a method
of applying force to pull towards one another,
sections which have been forced apart. Here the
reverse takes place and an external pull is applied
to pull apart or draw outwards areas that have been
pushed or drawn in towards each other. The combination
used to obtain this external pull employs
toes, links and clamps on the end of the push ram
and extension tubes. Two types of clamps are used,
the pull clamps for attachment to flat edges and the
wing clamps which have deep throats or lipped
edges. Both clamps have an alligator type of action
and are first tightened down on the centre bolt until
the jaws are parallel and in contact with the surface
of the panel to be gripped. Pressure is then applied
by tightening the rear bolt, which cants the jaws
forward and causes them to bite into the surface of
the metal. It may well be that the time taken to set
up this combination is greater than the time to pull
out the damage once it is in position. On flat
panels where it is not possible to get at the edges
or where there is not an edge to locate a clamp, the
same result can be achieved by locating the toes
in the bosses of a pair of solder plates which
have been sweated on to the panel. A roof panel
or car door is a typical example of this. To get at
both edges of a car door it is usually necessary
to remove the door, but this can be eliminated by
using a clamp at the free edge of the door and a
solder plate at the hinged edge. If properly used the
solder plates will withstand a pull of 13 800 kN/m2.
The plates should be sweated on with a layer of
body solder about 3 mm thick, using the minimum
of heat, and quenching with a wet rag to prevent
distortion. The same technique may be applied to
boot lids, front wings, bonnets and rear quarter
panels, and can be the means of repairing a panel
in a position where direct pressure from inside
would not have achieved a satisfactory result.
Figure 13.55Spreading with body jack (Blackhawk Automotive Ltd )
Figure 13.56Stretching with body jack (Blackhawk Automotive Ltd )
378Repair of Vehicle Bodies
Another combination, which is less popular in use,
is clamping (Figure 13.57).