Flat Belt Replacement
Has this happened to you?
It’s 4 pm and your ONE AND ONLY belt has come apart. You have no spare. Your customer needs what your machine cannot produce without a belt. In this scenario it is likely you end up with a delivery schedule you cannot meet, and a displeased customer.
Why would you tickle the dragon’s tail in this manner without considering this simple alternative:
Have a spare belt sitting on the shelf, and make a mental note to order another spare belt as soon as you reach for it. No stress, no down time, and no paying extra for overnight shipping. The takeaway here, - and we have a front row seat on this, - the 4 PM situation occurs more than you might think.
Sooner or later any machine that uses a flat drive belt is going to need a replacement flatbelt. In order to appreciate what may otherwise seem insignificant, we use this page to share useful information to elevate the flatbelt to the position it deserves so whatever machine that uses one can perform in the manner expected.
Belt Length Determination
The desirable method for determining the correct length of belt for your machine is to measure around the machine’s pulleys. If the machine has tension adjustment, the tensioning mechanism should be approximately centered.
For example, if the machine uses an idler pulley, the idler should be positioned roughly in the middle of its travel. If your machine tensions the belt by altering the distance between the pulleys that the belt will ride on, then that is the adjustment that should be centered. Leather is an exception to having the mechanism centered. Leather lasts a long time and if you are planning to use leather, your tension adjustment should be closer to the minimum because the leather will stretch over time and you will chase after it with the adjuster.
A good measuring device is a 50-feet tape measure because it is limp and will easily wrap around pulleys as opposed to one of the spring-return tape measures that are usually 12 feet or 25 feet. For applications that use small belts, a cloth tape measure will work.
Another way to determine what length belt you need is to calculate it. On the internet there are many engineering sites that will do this calculation for you. If you enter your pulley diameters and the center-to-center distance between them, the belt length will be given to you immediately.
If your machine has no adjustment for tension, leather is often used. This is because the leather stretches. We have made many leather belts for camel-back drill presses that have no tension adjustment. The customer must measure around the pulleys just like on other machines but then comes the tricky part. Before cutting the leather, I unroll about 4 feet of leather and test the stretchiness of it.
From that evaluation I estimate how much shorter to make the belt than the measurement that the customer gave me. The tension is built into the belt. On a measurement of about 12 feet, the belt will usually be about 2 inches shorter than the measurement around the pulleys. The belt will be installed diagonally across a pair of pulleys (on pulley cones) and then walked by hand into position onto the desired pulleys. So far, using this method, there have been only two or three times that the length was not quite right. Luckily, the belt was too long and I could shorten it. I generally charge another $10 to $20 for this shortening.
There have been situations where there was no adjustment for tension and leather was not suitable because it could not carry the load without stretching. One of the recommendations in a case like this is to have the customer measure around his crowned pulleys all the way out at the edges of the pulleys where the measurement would be the smallest. Then we would make a belt from a modern material that has almost zero stretch.
Some degree of difficulty might be encountered when installing the belt but when the belt arrives at the center of the pulleys it will be very tight.
If you are forced to measure a used belt to determine the length you need for a new one you must measure the inside circumference of the belt. This is how we measure belts when we make them. The belt is placed on a table or the floor and connected into a circle. The 50-feet tape measure is placed inside the belt and pushed out to make contact with the inside of the belt all the way around.
That way we are measuring the length that should equal the distance around the pulleys. If a belt is laid on the floor straight and measured instead of making it into a circle, the length will differ by an amount that is approximately equal to the belt’s thickness times 3.14.
For some applications (generally not lathes) we seek a certain amount of belt elongation for proper tension. Some belts are “happy” when they have been elongated by 1-2%. In cases like this, two pen marks are made on the belt prior to tensioning. These marks will be some known distance apart, such as 10 inches or 20 inches or whatever can be easily measured after installing the belt. Then, tension is applied and the distance from one mark to the other is measured. When the marks are 1% or 2% farther apart (or whatever we were seeking) then we know we have the tension right.
Belt Length Tolerance
Because belt material is not rigid, and because all belts will stretch to some extent, it is reasonable to tolerate a small amount of variation in the length of a belt. If a customer asks for a belt to be 62.25” long, we do our best to make the belt that length. However, if the belt length turns out to be 62” or 62.50”, it is still well within half a percent of the desired length, and deemed acceptable. Some belt makers express a tolerance of 1% on belts of about 5 feet in length. If there is adjustment for tension, the length is not as critical.
Generally, the tolerance increases as the belt length increases.
If a belt is used to connect two pulleys of equal diameter, any change in the length of the belt will change the center-to-center distance of the pulleys by half the amount that the belt length changed. If the length of a belt is very critical, it warrants a detailed discussion on the application.
Belt Run Direction
There are times when it is advantageous to run a flatbelt in a certain direction. Belts that have a laced joint are to be run in the direction that takes advantage of the beveled corners on the belt end that has one less staple than the other belt end. This reduces the possibility of the belt catching on guarding or other machine parts in close proximity. This direction is usually marked on the belt or belt installation instructions.
The rule for belts that have a glued joint is as follows:
The belt should be made up and applied to the pulley in such direction that the point of the lap on the outside of the belt is pointing away from the pulley it is approaching. This is quite old information and with the materials that many modern belts are made of, I would not worry about installing the belt with the joint pointing in the wrong direction. This run direction is also shown on the installation instructions.
For truly endless belts such as the woven ones, there is no run direction specified.
