Basics of belt drives

Power transmission belting has been used for more than 200 years. The first belts were

flat and ran on flat pulleys. Later, cotton or hemp rope was used with V-groove pulleys to

reduce belt tension. This led to the development of the vulcanized rubber V-belt in 1917.

The need to eliminate speed variations led to the development of synchronous or toothed

belts about 1950 and the later development of fabric-reinforced elastomer materials.

Today, flat, V, and synchronous belting is still being used in power transmission. When

compared to other forms of power transmission, belts provide a good combination of

flexibility, low cost, simple installation and maintenance, and minimal space

requirements.

Belt-driven equipment uses readily available components. Replacement parts can be

easily obtained from local distributors. This availability reduces downtime and inventory.

Sheaves and pulleys are usually less expensive than chain drive sprockets and have little

wear over long periods of operation.

Belt types

All power transmission belts are either friction drive or positive drive. Friction drive belts

rely on the friction between the belt and pulley to transmit power. They require tension to

maintain the right amount of friction. Flat belts are the purest form of friction drive while

V-belts have a friction multiplying effect because of wedging action on the pulley.

Positive drive or synchronous belts rely on the engagement of teeth on the belt with

grooves on the pulley. There is no slip with this belt except for ratcheting or tooth

jumping.

Flat belts

Modern flat belts are made with reinforced, rubberized fabric that provides strength and

high friction levels with the pulley (Fig. 1). This eliminates the need for high tension,

lowering shaft and bearing loads. Flat belts can transmit up to 150 hp/in. at speeds

exceeding 20,000 fpm.

Fig. 1. Flat belts have thin cross-sections and wrap around pulleys easily

A significant advantage of flat belts is efficiency of nearly 99%, about 2.5-3% better than

V-belts. Good efficiency is due to lower bending losses from a thin cross-section, low

creep because of friction covers and high modulus of elasticity traction layers, and no

wedging action into pulleys.

Pulley alignment is important to flat belts. Belt tracking is improved by crowning at least

one pulley, usually the larger one. Flat belts are forgiving of misalignment; however,

proper alignment improves belt life.

Different flat belt surface patterns serve various transmission requirements. In high-

horsepower applications and outdoor installations, longitudinal grooves in the belt

surface reduce the air cushion flat belts generate. The air cushion reduces friction

between the pulley and belt. The grooves nearly eliminate the effects of dirt, dust, oil, and

grease and help reduce the noise level.

Flat belts operate most efficiently on drives with speeds above 3000 fpm. Continuous,

smooth-running applications are preferred. Speed ratios usually should not exceed 6:1. At

higher ratios, longer center distances or idlers placed on the slack side of the belt create

more wrap around the smaller pulley to transmit the required load.

V-belts

Fig. 2. V-belts come in a wide variety of sizes and shapes

V-belts are commonly used in industrial applications because of their relative low cost,

ease of installation, and wide range of sizes (Fig. 2). The V-shape makes it easier to keep

fast-moving belts in sheave grooves than it is to keep a flat belt on a pulley. The biggest

operational advantage of a V-belt is the wedging action into the sheave groove. This

geometry multiplies the low tensioning force to increase friction force on the pulley

sidewalls (Fig. 3).

Fig. 3.

Classical V-belts are frequently used individually, particularly in A and B sizes. The

larger C, D, and E sizes generally are not used in single-belt drives because of cost

penalties and inefficiencies. Multiple A or B belts are economical alternatives to using

single-belt C, D, or E sections.

Narrow V-belts, for a given width, offer higher power ratings than conventional V-belts.

They have a greater depth-to-width ratio, placing more of the sheave under the

reinforcing cord. These belts are suited for severe duty applications, including shock and

high starting loads.

Banded V-belts solve problems conventional multiple V-belt drives have with pulsating

loads. The intermittent forces can induce a whipping action in multiple-belt systems,

sometimes causing belts to turn over. The joined configuration avoids the need to order

multiple belts as matched sets.

Banded V-belts should not be mounted on deep-groove sheaves, which are used to avoid

turnover in standard V-belts. Such sheaves have the potential for cutting the band of

joined belts. Extremely worn sheaves produce the same result.

V-ribbed belts combine some of the best features of flat belts and V-belts. The thin belt

operates efficiently and can run at high speeds. Tensioning requirements are about 20%

higher than V-belts. The ribs ensure the belt tracks properly, making alignment less

critical than it is for flat belts.

Synchronous belts

Synchronous belts have a toothed profile that mates with corresponding grooves in the

pulleys, providing the same positive engagement as gears or chains. They are used in

applications where indexing, positioning, or a constant speed ratio is required.

The first tooth profile used on synchronous belts was the trapezoidal shape (Fig. 4). It is

still recognized as standard. Recent modifications to tooth profiles have improved on the

original shape. The full-rounded profile distributes tooth loads better to the belt tension

members. It also provides greater tooth shear strength for improved load capacity.

Fig. 4. Synchronous belts have several tooth shapes

A modified curvilinear tooth design has a different pressure angle, tooth depth, and

materials for improved load/li fe capacity and nonratcheting resistance.

Synchronous belts can wear rapidly if pulleys are not aligned properly, especially in long-

center-distance drives, where belts tend to rub against pulley flanges. To prevent the belt

from riding off the pulleys, one of them is usually flanged. A recent development has

produced a belt and pulley that use a V-shaped, instead of straight, tooth shape. It runs

quieter than the other shapes and doesn't require pulley flanges.

Undertensioning causes performance problems. The drive may be noisy because belt

teeth do not mate properly with pulley grooves or the belt may prematurely wear from

ratcheting. High forces generated during belt ratcheting are transmitted directly to shafts

and bearings and can cause damage.

Link belts

Link-type V-belts consist of removable links that are joined to adjacent links by shaped

ends twisted through the next link (Fig. 5). With this design, belts can be made up of any

length, reducing inventory. The belts are available in 3L, A/4L, B, C, and D widths in

lengths from 5 to 100 ft.

Fig. 5. Link-type belts are used to make instant V-belt replacements

These belts can transmit the same horsepower as classic V-belts. The links are made of

plies of polyester fabric and polyurethane that resist heat, oil, water, and many chemicals.

Advantages of link belts include quickly making up matched sets, fast installation

because machinery doesn't have to be disassembled, and vibration dampening.

Disadvantages include cost and the possible generation of static charges. The belt should

be grounded when used in high-dust applications.

Alignment

Misalignment is one of the most common causes of premature belt failure (Fig. 6). The

problem gradually reduces belt performance by increasing wear and fatigue. Depending

on severity, misalignment can destroy a belt in a matter of hours. Sheave misalignment

on V-belt drives should not exceed 1/2 deg. or 1/10 -in. of center distance. For

synchronous belts it should not exceed 1/4 deg. or 1/16-in. of center distance.

Fig. 6. Improper drive maintenance is the biggest source of belt drive problems

Angular misalignment (Fig. 7) results in accelerated belt/sheave wear and potential

stability problems with individual V-belts. A related problem, uneven belt and cord

loading, results in unequal load sharing with multiple belt drives and leads to premature

failure.

Angular misalignment has a severe effect on synchronous belt drives. Symptoms such as

high belt tracking forces, uneven tooth/land wear, edge wear, high noise levels, and

potential failure due to uneven cord loading are possible. Wide belts are more sensitive to

angular misalignment than narrow belts.

Fig. 7. Misalignment causes belt wear, noise and excessive temperatures

Parallel misalignment also results in accelerated belt/sheave wear and potential stability

problems with individual belts. Uneven belt and cord loading is not as significant a

concern as with angular misalignment.

Parallel misalignment is typically more of a concern with V-belts. They run in fixed

grooves and cannot free float between flanges to a limited degree as synchronous belts

can. Parallel misalignment is generally not a critical concern with synchronous belts as

long as the belt is not trapped or pinched between opposite sprocket flanges and tracks

completely on both sprockets.

Tension

Total tension required in a belt drive depends on the type of belt, the design horsepower,

and the drive rpm. Since running tensions cannot be measured, it is necessary to tension a

drive statically.

The force/deflection method is most often used. Once a calculated force is applied to the

center of a belt span to obtain a known deflection, the recommended static tension is

established. Most design catalogs provide force and deflection formulas.

With too little tension in a V-belt drive, slippage can occur and lead to spin burns, cover

wear, overheating of the belt, and possibly overheating of bearings. Not enough tension

in a synchronous belt causes premature tooth wear or possible ratcheting that will destroy

the belt and could break a shaft.

When installing a new belt, installation tension should be set higher. Generally 1.4-1.5

times the normal static tension. This is necessary because drive tension drops rapidly

during the seating-in process. This extra initial tension does not affect bearings because it

decays rapidly.

Plant Engineering magazine extends its appreciation to The Goodyear Tire & Rubber

Co. for its cooperation in making the cover photo possible.

Belt application matrix

Application Synchronous belt V-belt

V-

ribbed

belt

Polyurethane Rubber Double-

sided

Heavy-

duty

Light-

duty Polyurethane

Speed/Load

High speed 2 2 1 1

Low speed 1 1 2 3

High load 1 2 4 3 3

Low load 1 2 3 4 4

Shock/impulse load 3 4 1 2

Application Synchronous belt V-belt

V-

ribbed

belt

Serpentine drive 1

Serpentine drive w/

shock load 2

Twisted drive 1 2 3

Clutching drive 1 2

Index drive w/high

load 1

Index drive w/low

load 1 2

Drive

characteristics

Reversing direction 1 1 3 4 2

Frequent start/stop 1 1 3 4 2

Start under load 1 2 3

Smooth running 3 2 1 1

Variable speed 1

Oil, chemical

environment 1 3 4 2

High temperature 1 2 4 4 3

Low temperature 1 2 3 4

1=First choice, 4=Last choice Chart courtesy The Gates Rubber Co.

Troubleshooting V-belt drives

Problem Cause Remedy

Belt stretch beyond take-up

Belts stretch unequally

Misaligned drive

overloading some belts.

Belt tensile member

broken from improper

installation

Realign and retension drive. Replace

with a new, matched set, properly

installed

All belts stretched

equally

Insufficient take-up

allowance

Check take-up and follow

recommended allowance

Greatly overloaded or

under-tensioned drive Redesign drive

Short belt life

Rapid belt failure Tensile members damaged

from improper installation

Replace with new, matched set,

properly installed

Worn sheave grooves Replace sheaves

Under-designed drive Redesign drive

Belt sidewalls soft and

sticky. Low adhesion

Oil or grease

contamination of

Remove source of oil or grease. Clean

belts and sheave grooves cloth

Problem Cause Remedy

between cover, plies.

Cross section swollen

belt/sheave moistened with nonflammable, non-

toxic degreasing agent or commercial

detergent and water

Belt sidewalls dry and

hard.

High-temperature

environment Remove source of heat

Low adhesion between

cover and plies Ventilate drive

Deterioration of belt's

rubber compounds Belt dressing

Never use dressing on rubber V-belts.

Clean belts and sheave grooves cloth

moistened with nonflammable, non-

toxic degreasing agent or commercial

detergent and water. Tension drive

properly to prevent slip

Extreme cover wear Belts rubbing against belt

guard or other obstruction

Remove obstruction or align belts to

provide proper clearance

Spin burns on belt Belts slip on starting or

load stalls Retension drive

Bottom of belt cracked Sheaves too small Redesign drive for larger sheaves

Broken belts Object falling into or

hitting drive

Replace with new, matched set of

belts

Belt turnover

Excess lateral belt whip Use banded belt

Foreign material in sheave

grooves Remove material. Shield drive

Misaligned drive Realign drive

Worn sheave grooves Replace sheaves

Tensile member broken

from improper installation

Replace belts with new, matched set,

properly installed

Incorrectly placed idler

pulley

Carefully align idler pulley on slack

side of drive, as close as possible to

driver sheave

Belt noise

Belt slip Retension drive

Improper driven speed

Incorrect driver/driven

ratio Design error Change sheaves

Hot bearings

Drive overtensioned

Worn sheave grooves.

Belts bottom out and can't

transmit power unless

overtensioned

Replace sheaves. Tension drive

properly

Improper tension Retension drive

Sheaves too small

Motor/belt manufacturer's

recommendations not

followed

Redesign drive

Problem Cause Remedy

Bearing wear Underdesigned bearings or

poor bearing maintenance

Observe recommended design and

maintenance

Drive undertensioned Belts slip and cause heat

buildup Retension drive

Power transmission belting manufacturers

The following companies provided input for this article by responding to a written

request from Plant Engineering magazine. For more information on their product lines,

circle the number on the Reader Service Card or visit their web site.

Circle Company Belt type Horsepower

range

Speed range,

fpm

Max.

length, in.

221 Fenner Drives V 1/16—6 275—600 none

fennerindustrial.com Flat 0.01—0.1 98—196 none

Link varies by application

222 Emerson Power V 1.3—925 1000—6500 450

emerson-ept.com Synchronous 3.8—318 1000—6500 270

Link 1.3—16 1000—5000 450

223 The Gates Rubber Co. V 0.1—1000 1—20,000 663

gates.com Synchronous 0.1—1200 1—15,000 270

Flat 0.1—50 1—25,000 126

Link 0.1—50 1—7000 none

224 Goodyear Tire & Rubber

Co. V 0—1000 0—10,000 900

goodyearptp.com Synchronous 0—1100 0—20,000 280

Flat 0—500 0—10,000 1620

226 Shingle Belting Co. V 4—16 1000—5000 open

Flat 1—20 1000—8000 open

225 Stock Drive

Products/Sterling Instr. V 0.1—4.5

500—12,000

rpm 32.5

sdp-si.com Synchronous 0.01—18 8000—

25,000 rpm 149.6

Flat 0.04—0.2 2000—

20,000 rpm 19.7

Belt drive advantages

• Cleanliness

• Lubrication-free

• Absorbs shock loads

• Wide selection of speed ratios

• Can provide variable speeds

• Quiet operation

• Efficiency over 95%

• Transmits power between widely spaced shafts

• Visual warning of failure

Belt drive disadvantages

• Need to retension periodically

• Deterioration from exposure to lubricants or chemicals

• Cannot be repaired, must be replaced