Chapter 8 Power Tools 195
Copyright Goodheart-Willcox Co., Inc.
Remember that the purpose of a jack is to lift heavy objects, not to hold
them in place. Never trust a jack alone to hold up a vehicle that must be
serviced from the underside. Use a jack stand, which is designed for stability,
or cribbing constructed from short timbers to hold vehicles up while working
underneath them. See Figure 8-20. The height of jack stands can be adjusted
using a mechanical device such as a screw or pin.
Cribbing is a method of stacking timbers in a
crisscross pattern to form a structure resembling an
old-time corn crib, Figure 8-21. Using cribbing is a
stable way to temporarily support heavy equipment
while it is under construction or in for repairs.
Hydraulic presses are sturdily framed machines
that use a hydraulic cylinder to apply pressure to
objects for a variety of purposes. The most basic types
use a bottle jack mounted in an H-shaped frame.
These are designed for pressing bushings and bearings
and for moderate bending of metals. Basic hydraulic
presses may be operated with a manual pump or
with an electric or pneumatic pump. Many include
a pressure gauge to monitor the force exerted by the
press. One common use of a hydraulic press is to
perform testing on the strength of welded objects.
Sophisticated versions of hydraulic presses are
used in the metalworking industry to shear, punch,
bend, and cold-form metal. Figure 8-22 shows one
type of machine. A small version of these heavy-duty
machines can exert forces in the range of 40 to 60 tons.
Another type of hydraulic press is designed for
bending pipe and tubing. Pipe benders may be stationary, or they may
be portable devices that can be taken to the job site. Both types use rollers
to press pipe or tubing around a mandrel—a curved piece or form that
determines the radius of the bend. Rollers and mandrels are matched sets
B
Mikhail Abramov/Shutterstock.com
Figure 8-19. Simple hydraulic jacks put Pascal’s law into practical use. A—The portable
design of a bottle jack makes it ideal for agricultural field work. B—The low profile design
and durable wheels of a typical floor jack make it easy to slide under vehicles and equipment.
A
sizov/Shutterstock.com
ericlefrancais/Shutterstock.com
Figure 8-20. After agricultural equipment is lifted
with a mechanical or hydraulic jack, jack stands are
moved into place and used to support the heavy load.
Never rely on mechanical or hydraulic jacks alone
to support a load for an extended period of time.