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The
Printing Industry's Premier Supplier of |
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PRINTER'S PARTS OF LOS ANGELES |
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The
Printing Industry's Premier Supplier of |
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PRINTER'S PARTS OF LOS ANGELES |
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Printer's Parts of Los Angeles and San Diego |
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From the Service Department of SDPP |
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Volume
1 |
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THE
PRINTING PRESS DRIVE TRAIN V-Belts,
like other moving parts on the press, are subject to wear.
While they are manufactured with a combination of rubber and cord
for extra strength and durability, they will eventually stretch, become
frayed. loose their resilience, and become ineffective by losing their
gripping ability. Damage is
also caused by the belts coming into contact with lubricants. It is important for the press operator to realize Regardless of the manufacturer, most belts can be cross referenced by the
numbers appearing on the belt. Undoubtedly,
some manufacturers produce a better belt than others.
However, belts are also classified for job ratings by the quality
of the manufacturing. The
important thing is that you use a quality belt intended for your machine. When belts need replacing, always replace them in sets to assure uniform
performance from each belt. When
installing the new belts, use the adjustments provided to relieve and
apply pressure. Don’t damage the belt by stretching it over the sheave.
It should be adjusted and reseated in the sheave.
Belt tension should be checked carefully, as over tightening will
cause serious stress on the drive motor from overheating.
Bearings are used in the side frames of the press to help provide
lubrication and the shafts run true.
On small presses, shafts used to support and rotate using pulleys
can be flexed and cause a bind by over tightening belts.
This is a common occurrence. Other than creating a bind, proper tension is needed on V belts to
correctly contact the sheave. Unlike the round belt, or flat belt, all torque is directed
against the side of the sheave. The
taper of the belt must seat firmly against the taper of the sheave.
The belt is not intended to contact the bottom of the sheave.
The contact would lessen the effective action by relieving pressure
on the sides, lessening the gripping action.
(Stretched or worn belts). The
seating of the belt in the sheave should allow the belt to ride just
slightly above the pulley. Before making the final adjustment, rotate the handwheel of the machine to
allow the belts to seat evenly in both the driving and driven sheaves.
At the same time, check to be certain both sheaves are in perfect
alignment. Misalignment
alignment of pulleys create problems for any belt driven system, but even
more for v belts and sheaves. Checking
Belt Tension Mechanics all have their own method for checking belt tension.
Some use a fist to hit the belt, using the reflex to determine the
tension. Others use a 1/2
inch give in the belt tension from both sides of the belt when squeezed
together to adjust the belts. The
important thing is V-Belts need a tension that will eliminate slack in the
belt. Over tensioning will cause problems. Care and maintenance of V belts other that adjusting is almost non
existent as previously stated. The
largest problem is contact with lubricants.
There are no special cleaners, other than wiping clean with a dry
rag. To our knowledge, no
belt cleaners are formulated and sold, and belt dressing is not
recommended. Wear and
replacement of V belts on printing presses is largely overlooked by press
owners and operators. They do
make a difference in a smooth operating drive train. Speed
Control Many newer small offset machines are now using speed controllers which are
tied into the electrical system though a electronic board, and controlled
from a central panel. But,
many still use the basic variable speed pulley. The variable speed is a universally used, un-complicated system of raising
and lowering the speed by driving a turning shaft with different diameter
pulleys. The pulleys used are
tapered sheaves driven by V-belts to control the position with respect to
the depth the belts will ride. As with other sheave type pulleys, the V-belts are tapered to correspond
to the pulley using the torque created by the side of the belt rather than
the bottom. Instead of one
piece, it is manufactured as two separate side plates.
The halves are joined by a shaft and held tightly together by a
compression spring. By
raising and lowering the motor mounting plate with either a lever or a
crank, the tapered belt will lower into the pulley and separate the plates
as tension is put on the belts. The
spring pressure between the two pulley halves will take up the slack in
the belt and force the belts to ride higher as the tension is relieved
from the belts. Normally the variable speed pulley is fastened directly to the shaft of
the motor, directly driving another pulley. When the belts are riding high
it takes on the characteristics of a larger pulley, changing the drive
ratio between the pulleys - under
most circumstances, increasing the speed.
As the belts drop lower, it becomes
a smaller circumference and lessens the speed.
Obviously, the speed is changed in increments by the crank or
lever. however, the increment posted on the press does not
necessarily reflect the speed. The
variable speed is universally used on all types of machinery.
It is economical for manufacturers to install on machinery. It is simplistic and works well, but when used under
stressful conditions such as a printing press, it needs maintenance. Most
pulley halves use a key and key way slot to mount the pulley halves in
alignment on the shaft. The
halves need to slide on the shaft as the belt raises and lowers into the
pulley. If a bind develops
because of lack of lubrication, the belt forces the pulley apart - causing
vibration and strain on the motor plate and belts.
It also becomes noisy. The
belts produce heat, causing the un-lubricated pulley to form rust, which
will wear away the key and enlarge the key way.
When this damage is allowed to go unchecked, the pulley will become
ineffective and need to be changed. The
binding pulley halves will also pinch the belt causing premature wear.
Changing the speed of the press - when not in operation, forces the
belt into the pulley rather than using a turning motion to allow the belts
to seat into the halves. Improper
adjustments of the motor plate when attempting to regulate the speed is
another source of problems. The
belt should never bottom out to the point that it supports the weight of
the motor. This causes
vibration and noise. A belt
riding too high on the pulley will cause loss of torque.
Always use the manufactures speed recommendations. Certain
pulleys are equipped with grease fitting, but are usually hard to access.
Belts should not be exposed to grease.
The most common way used to lubricate the pulley is to separate the
halves with a screwdriver. Spray
the shaft with a lubricating oil, the work the halves several times to be
certain they are sliding freely on the shaft.
If you decide to remove the pulley from the motor and disassemble
it, to inspect for wear, care needs to be taken.
The coil spring is usually compressed on the shaft tightly.
When the clip or fastener is removed, the spring can fly. When
replacing the pulley on the motor, tighten it securely on the flat of the
shaft to prevent slipping. |
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