A lesson in belts and their importance to machines


People sometimes ask about flat belts. A belt can be defined as being a continuous strip of some flexible material placed around two pulleys under a certain amount of tension to transmit power from one pulley to the other.

This definition also describes chain, rope and V-belts, but we’ll stick to flat belts as used to power many farm machines such as threshers, balers, hammer mills, husker-shredders and silo fillers.

Belt types

Belts were made of leather, rubber or canvas. Leather was considered the best, but was the most expensive and required more care. Double thickness leather belts were made for heavy duty use on large diameter pulleys but weren’t flexible enough for smaller pulleys. Few leather belts are seen today.

Canvas belts

Canvas belting is made of several layers of canvas stitched along both edges as well as several times down the center. Canvas is the cheapest and usually is treated with oil or painted to make it waterproof.

A 4-ply canvas belt is considered as strong as single ply leather. Probably the most common belt found today is the rubber belt. Made with a foundation of several layers of cotton ducking to which a rubber compound is applied and vulcanized, rubber belts are impervious to steam and water and will not slip as readily as leather belts. 3-ply rubber and single ply leather belts have equivalent strengths.

Rubber belts

Endless rubber belts are often spliced by stitching before being vulcanized, while leather belts may be spliced by tapering the thickness of each end and then overlapping and cementing those ends together.

The two ends of an endless belt were often joined by hand lacing with leather thongs or very pliable wire, although “store-bought” metal lacing is probably more popular today, as few folks are around anymore that know how to hand lace the things.

Metal lacing belts

The metal lacings consist of a series of loops that are applied to each end of the belt by either hammering them into the belt material or using a special machine to press them into place.

The belt ends are brought together so the loops interlace and a rawhide or steel hinge pin is inserted through the loops to hold them together. This method of connecting has the advantage of being easy to disconnect, along with being quite flexible.

Leather belts

Leather belts should be run with the hair (smooth) side toward the pulley and must be kept clean and flexible. The old belt dressing contained a good deal of rosin, along with cod-liver and neat’s-foot oil (I looked up the ingredients in modern spray-on belt dressings and it’s a bunch of obscure chemicals which I don’t recognize).

Rubber belts shouldn’t need any dressing as they shouldn’t slip if tight enough. In fact, sticky belt dressings have a tendency to pull off the outer layer of rubber and hasten the deterioration of a rubber belt.

All belts should be kept clean and free of oil and grease. Leather belts should not be allowed to get wet.

When a belt is running under load, there is a tight and a slack side to the belt preventing 100% of the power to be transmitted. The tight side stretches under the tension, while the slack side contracts, resulting in belt creep which is normal and is not the same as belt slip due to improper tensioning of the belt.

No slippage

Belts should be run only tight enough to prevent slippage. A long belt doesn’t need to be as tight as a short belt due to its weight. If a belt is run too tight it puts undue strain on the belt, pulleys, shafts and bearings. A too loose belt will lose power through slippage and has a tendency to flap and be thrown off if a sudden load is encountered.

In the old days of steam engines, the threat of fire from exhaust sparks was serious and belts were long, as much as 150 feet, in order to keep the engine as far from the separator as possible.

With gas or oil tractors the possibility of exhaust sparks is much less and a 50-foot belt is plenty long enough; I’ve often run a thresher with a 50-foot belt, and short belts are commonly used on hammer mills and silo fillers.

The direction in which the machine must be run, as well as the direction of the driving pulley must be taken into consideration.

If necessary, the direction may be reversed by putting one twist in the belt, which also helps to keep a long belt from flapping in a cross wind, however an uncrossed belt is more efficient as both heat and friction is generated at the point where the two sides of a crossed belt rub together.

Engine is important

In the old days, the thresherman always tried to set his separator so the wind came from the direction of the engine, preferably with a little “quartering.” This lessened the effects of a cross wind on the belt and kept the straw and chaff from being blown back onto the crew, while at the same time it blew the smoke and hot ashes from the engine away from the separator at an angle.

The velocity or speed, the width, and the amount of tension all determine the amount of power that can be transmitted by a belt. The length affects it as well, as a longer belt is heavier and thus requires more power.

Drive pulley

It’s very important that the driving and driven pulley are accurately aligned to prevent the edges of the belt from running partially off the inside of the pulleys and being damaged by rubbing against some part of the machinery, or running completely off the outside of the pulleys.

A handyman type jack can be a convenient way of moving the front of the tractor to one side or the other in order to attain perfect alignment.

In the threshing chapter of Implements for Farming with Horses and Mules, I said: “A little caution and good judgment, along with a lot of practice, are required to quickly and properly line up and set a tractor and belt-driven machine.”

And that’s all I know about flat belts and their use.

Get our Top Stories in Your Inbox

Next step: Check your inbox to confirm your subscription.



We are glad you have chosen to leave a comment. Please keep in mind that comments are moderated according to our comment policy.

Receive emails as this discussion progresses.