Sheave misalignment is another problem to watch for during routine inspection.  While the misalignment illustrated in Fig. 1 is exaggerated, it does indicate another common cause of shortened belt life.

Sheaves are usually out of line because the drive and driven shafts are not parallel.  However, even perfectly parallel shafts do not guarantee sheave alignment.

Fig. 1 shows the most common types of misalignment – all caused by the shafts not being parallel.

Possibly the best way to check alignment is to use a straight edge between the sides of the sheaves (Fig. 2).

The straight edge should touch the sheaves at the four arrows.  Then rotate each sheave and note whether the contact of either sheave with the straight edge is disturbed.  If so, a shaft is bent or a sheave is wobbling.

Another method is to sight across the edge of the sheave groove in line with the belt.  Harmful misalignment will show up as a bend in the belt at the point where it enters or leaves the sheave grooves.

V-belts, because of their flexibility, can tolerate more misalignment than other types of power drives.  But there is a point beyond which they should not be misaligned if they are to deliver the service they are designed to give.

ILLUS. NO. PS8669

Fig. 1 – Misaligned Sheaves

ILLUS. NO. PS8670

Fig. 2 – Check Alignment of Sheaves

In multiple drives, the belts must be accurately matched so that the load is evenly distributed among the belts.  If the load isn"t evenly distributed, the shorter belts will do all the work.

Fig. 3 shows mismatched belts.  This drive has seven new belts, but one (arrow) is getting a "free ride."  Although all belts are new, the outside belt is obviously longer and not pulling its share of the load.

When watching for possible mismatching, look for sagging belts on the tight side of the drive – the side on which the belts approach the motor.  If they are even on this side (though not even on the slack side) the belts are pulling their fair share of the load.

To ensure proper belt matching, V-belts are marked with belt size numbers.

ILLUS. NO. PS8671

Fig. 3 – Mismatched Belts

All belts and sheaves wear to some degree after they have been in use.  As wear occurs the belts ride lower in the sheave grooves.  Since center distance has to be increased to compensate for the wear, a worn belt is "longer" than a new belt.  (The worn belt also stretches in use.)

It is important, therefore, to replace the entire set on multiple-belt drives even though only one belt might fail.  If only one belt is replaced, the replacement will in effect be "shorter" than the other belts.

In Fig. 4 only the broken belt has been replaced.

Because the old belts are worn and stretched, the new belt is pulling most of the load.  And, since seven belts are obviously needed to meet the horsepower requirements of the application, the overload will considerably shorten the life of the new belt.

ILLUS. NO. PS8672

Fig. 4 – Replacing Only One Belt In A Set Means Overloading The New Belt And Underloading The Others

In Fig. 5 all belts in the set have been replaced and each belt is carrying an equal share of the load. When belts pull together like this, they"ll give the service they"re designed to provide.

Replacing all belts on a multiple drive when only one has failed is insurance against forced downtime and lost production.

ILLUS. NO. PS8673

Fig. 5 – Replace All Belts In A Set At One Time As Shown

Too little tension will cause slippage or slip-and-grab, causing the belt to break.  If the belt does not break, slip will cause excessive cover wear, burned spots, and overheating.

Too much tension will cause belt heating and excessive stretch as well as damage to drive components such as sheaves and shafts.  The extra tightness will also place a heavier load on the bearings.

Remember that V-belts should ride on the sides of standard sheaves, not on the bottom of the groove.  Tension on a new belt should be watched during the first 24 hours of operation.  This is when the initial seating and stretch occurs.

Belt tension is often checked by deflecting the belt on one side with a spring scale (Fig. 6, top).  By measuring the deflection at a certain pounds pull, actual belt tension is found.  The recommended tension is given in the machine operator"s manual.

Another method is to depress the belt halfway between the sheaves using a ruler at right angles to a straightedge (Fig. 6, bottom).

ILLUS. NO. PS8674

ILLUS. NO. PS8675

Fig. 6 – Checking Belt Tension

In the four-groove sheave shown in Fig. 7, grooves (1) and (2) are worn much more than the others.  This wear allows the belts in the worn grooves to ride lower that the rest of the belts and creates the same effect as mismatched belts.

Also, it causes what is termed "differential driving" – a condition where the belts operate at different speed ratios between the drive and driven units.  In such a drive, the belts actually "fight" each other because some of them are trying to travel faster than others.

The wedging principle of V-belt drives makes it vital to keep the sheave groove sidewalls perfectly straight.

When the groove walls are dished out, the belt"s wedging action is reduced.  The gripping power is reduced and – in extreme cases – slippage occurs and belt life is greatly shortened.

When mismatched belts are spotted in a multiple drive, the sheave grooves should be examined for excessive wear.  On banded V-belts, if the tie band is separating from the belts, this may also mean worn sheave grooves.

Because of slight variation in V-belts and sheave grooves, the riding position of new belts in new sheaves will vary.  In a standard sheave, it will vary from 1/16-inch (1.6 mm) above the top of the grooves to 1/16-inch (1.6 mm) below the top (Fig. 8).  However, for a given multiple-groove sheave, the riding positions of the belts should be uniform.

A new belt that rides lower than the other belts in a drive may mean that the sheave groove sidewall is "dished out" (Fig. 9).

ILLUS. NO. PS8676

Fig. 7 – Irregular Sheave Grooves

ILLUS. NO. PS8677

Fig. 8 – Correct Position Of V-Belt
In Sheave Groove
(R.M.A. Standard Sheave)

SHINY SHEAVE GROOVE BOTTOMS

In standard sheaves, a shiny groove bottom is a sign that the belt, or sheave, or both are badly worn and the belt is bottoming in the groove.  This causes the belt to lose its grip and waste power.

Worn sheaves or shiny sheave groove bottoms will usually show up first on the smaller sheave of the installation.

ILLUS. NO. PS8678

Fig. 9 – Sheave Grooves
"Dished Out" By Wear
(Replace Sheaves)

WOBBLING SHEAVES

Wobbling sheaves are another cause of shortened belt life.  A wobbling sheave (Fig. 10) whips the belt from side to side, subjecting it to a lateral strain which produces rapid sheave and belt wear.  A wobbling sheave also transmits vibrations to the machine.

ILLUS. NO. PS8679

Fig. 10 – Wobbling Sheave

DAMAGED SHEAVES

Sometimes sheaves are damaged as shown in Fig. 11.  Cast sheaves may become chipped; pressed steel sheaves can get bent.

Regardless of the type of damage to the sheave, damaged sheaves always produce rapid belt wear.  The only way to prevent short belt life is to correct the sheave condition immediately.

ILLUS. NO. PS8680

Fig. 11 – Damaged Sheaves

OVERHEATED BELTS

V-belts are composed mainly of rubber – and heat is the natural enemy of rubber.

When the drive is open or ventilated, the temperature is usually not sufficient to affect the belts.

Under field conditions, belts operating in temperatures of less than 180°F (84°C) are not materially affected.  Temperatures in excess of 180°F (84°C) may harden the belt, causing it to crack and stretch.  To prevent this, be sure that screens and shields are kept clean for full ventilation.

Fig. 12 shows a belt damaged by excessive heat.  It appears cracked or checked on the bottom edge and should be replaced.

ILLUS. NO. PS8681

Fig. 12 – Belt Ruined By
Too Much Heat

OILY OR GREASY BELTS

Oil or grease, if allowed to soak into the rubber, (Fig. 13) causes a V-belt made of natural rubber – or synthetics – to become soft, to swell, and to deteriorate very rapidly.

When oil or grease is found on a belt, wipe it off with a clean cloth dampened with a detergent solution.  Then wipe dry with a clean dry cloth.

For naturally oily or greasy operations, oil-resistant belts can be installed.

CAUTION:  Never try to clean a belt while it is operating.

ILLUS. NO. PS8682

Fig. 13 – Belt Ruined By Oil
And Grease Soaking

Storage on The Machine

If a machine is stored for a long period, observe these rules:

·        If belts are not be removed, relieve belt tension by loosening all tighteners (Fig. 17).  This is necessary to prevent the belt from "setting" or developing unequal stresses that might lead to early failure.

·        If possible, remove all belts.  Thoroughly clean them, then store in a cool, clean, dry place.

·        If the belts are removed, coat sheave grooves with an anti-rust compound or grease prior to storage.  Be sure to remove such rust preventives before installing the belts and starting the machine.  Sheave grooves can also be protected with a section of discarded belt tied in place.

·        Protect all movable or sliding parts of variable-speed drives by lubricating thoroughly to prevent corrosion due to moisture.

ILLUS. NO. PS8686

Fig. 17 – Loosen All Belts If Stored On The Machine In Off Seasons

Trouble Area
and Observation
Banded belt with
one strand riding
outside sheave
grooves (Fig. 19).
NOTE:  Distinct
groove in side-
wall of outside belt.

Cause
Possible mis-
alignment, lack
of tension or
foreign object
forced belt
from sheave
grooves.

Remedy
Properly align
drive, retension
and remove
any interference.

ILLUS. NO. PS8688

Fig. 19 – Banded V-Belt Riding Outside Of Sheave Grooves

Trouble Area
and Observation
Belts have
separated com-
pletely from tie
band (Fig. 20).

Cause
Riding outside
of sheave
grooves.

Remedy
Proper
maintenance of
drive with
installation of belt.

ILLUS. NO. PS8689

Fig. 20 – Separation Of All
Belts From Band

Trouble Area
and Observation
Bottom of belts
cracking
(Fig. 21).

Cause
Belt running on
too-small
sheaves.  Belt
slipping caus-
ing heat build-
up and gradual  
hardening of
undercord.
Extended use.

Remedy
Consider mini-
mum diameters
for drives,
drive and idler
sheaves, and
pulleys.

Check belt
tension.

ILLUS. NO. PS8690

Fig. 21 – Bottom Of Belts Cracking

Trouble Area
and Observation
Top of tie
band frayed
or damaged.
(Fig. 22).

Cause
Obstruction on 
machine
interfering
with normal
operation of
belt.

Remedy
Realign drive
and remove
obstruction.

ILLUS. NO. PS8691

Fig. 22 – Top Of Band

Frayed Or Damaged

Trouble Area
and Observation
Tie band
separating
from belts
(Fig. 23).

Cause
Worn sheaves.

Remedy
Gauge sheave
grooves and
replace with
standard-groove
sheaves.

ILLUS. NO. PS8692

Fig. 23 – Band  Separating From Belts

Trouble Area
and Observation
Large holes or
blisters appear
on tie band
(Fig. 24).

Cause
Trash and
foreign
material
accumulating
between belts.

Remedy
Shield drive.

ILLUS. NO. PS8693

Fig. 24 – Holes Or Blisters
In Tie Band

An improperly aligned tightener pulley caused this flat drive belt to climb the inner edge of the engine drive pulley, tearing the edge of the belt (Fig. 25).  One of these tears eventually caused the belt to tear completely through.

Check belt and pulley alignment to prevent unnecessary wear on the edge of the belt.

ILLUS. NO. PS8694

Fig. 25 – Flat Belt Tears –
Damaged Belt

The shiny appearance and the badly burned area (Fig. 26) were caused by operating this drive belt too loosely.  Note the notch in the edge of the belt (arrow) where it slid sideways against the flange of the drive pulley while slipping and burning the center section.  Similar damage can result from attempting to unload a machine with power without first cleaning it out.

Check belt tension periodically to prevent this type of belt damage.

ILLUS. NO. PS8695

Fig. 26 – Flat Belt Burn –
Damaged Belt