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Guidance on making Unimat drive belts

Many of our great customers frequently ask us how they can make their own Unimat SL and Unimat 3 drive belts. Here is the easiest way of preparing your own Unimat drive belts, with just a few, basic tools at home.


First thing to consider, is material from which belts can be made. There are two common options:

1) Black rubber, widely available O-rings (or seals) of which many different sizes are easy to source.


Rubber O-rings do not require welding. They come ready to use. In general, they are more soft and elastic than polyurethane. That is also exactly how they transfer torque from the motor: any rapid increase of torque will not be able to pass through rubber, because rubber will less or more absorb it. Just imagine you want to tow a car with bungee cord. Even if bungee cord is not very stretchable, it still does not seem to be a great idea. However, due to softness and high elasticity rubber effectively works better with small diameter pulleys, and does not slip on pulleys as much as harder grades of polyurethane belts.

If you have rubber O-rings, and think about using them for your Unimat, the first thing to do is find out your Unimat reference belt length, and then what your belt cut length is. Please see below to understand how you can get these values. Once the belt cut length is determined, by using a very simple tool you can easyli measure and verify suitability of your O-rings for Unimat belt application. 

2) Non-reinforced polyurethane cords


Polyurethane belts hardness and elasticity depends on the particular polyurethane hardness grade. The hardness grades reflect minimum pulley diameters with which the polyurethane cords might be effectively used. The softer grade, the smaller diameter pulley can be used. Harder grades work with bigger diameter pulleys, and can transfer the torque (from motor to spindle) without much dampening it. Also, non-reinforced polyurethane cord can be welded with ease, without any special equipment, providing very durable welds.  



When shape and size of the belt needs to be considered, the most important factor is the geometry of pulley:


Just from looking at the picture, you can almost immediately see that vee shaped groove pulley should not be used with round belts. The simple work principle of vee shaped groove is to provide wedging force which allows the belt to be firmly trapped in the groove. It is similar to work principle of hand pliers. If belt has got trapezoidal shape, it can be much more effectively wedged in the groove. If belt has got round shape, there is only small area between the vee shaped grove and the belt. This small area does not provide enough area for sufficient friction between the belt and pulley, and this is the reason why round belts tend to slip on Unimat SL vee groove pulleys.

We can only assume Emco ditched the idea of vee shaped groove pulleys, when Unimat 3 was designed. Plastic  spindle pulley now has got round profile. This improvement makes the possibility of belt slipping on the pulley... much more regular, and there is not much we can do about it. In vee shaped groove pulleys at least some wedging forces were able to trap the trapezoidal, or even round belt in the groove. The wedging forces were directly proportional to the belt's tension. For round belt in round shaped groove there is not any chance for wedging forces to exist.

If your plan is to use round belt with vee shaped groove pulley, there is one principle to remember: diameter of the belt must be large enough to provide some space indicated on the above drawing by red exclamation mark. This space allows wedging forces to work, and prevents belt from easy slipping. If your plan is to use round shape belts with round shaped groove pulleys then make sure the radius length of the belt is similar to the radius length of the groove. For Unimat 3 and Unimat SL 5mm diameter is usually regarded as a standard size. It is indeed probably the best size for most of your applications.


Cut lengths of polyurethane belts can either be calculated or established by simple measurements. 


The reference length is the first important value to determine the belt cut length, and is based on the following formula:



L - reference length

C - centre of first pulley shaft to centre of second pulley shaft distance

D - pitch diameter of large pulley

d - pitch diameter of small pulley

Unimat 3 motor-to-spindle belt pre-calculated length formula values:



Unimat SL motor-to-spindle belt pre-calculated length formula values:



Centre to centre pulley shaft distance for Unimat 3



Centre to centre pulley shaft distance for Unimat SL


Effective pulley diameter (EPD) or pulley pitch for Unimat 3 and Unimat SL spindle pulleys

for 5mm diam. round belt













Adjustment to the formula

The above formula applies ideally to flat belts on flat pulleys. To make this formula working for Unimat applications, where almost always round belts are used in either round, or vee shaped groove pulley, we have to adjust it in the following way:

a) replace value "D" with value "Z"

b) replace value "d" with value "z"


Z - large round belt pulley belt length diameter

z - small round belt pulley belt length diameter

Unimat 3 and Unimat SL spindle pulley "Z" values for 5mm diam. round belt




PJTOOLMAKERS motor double groove pulley "z" value

Cut length

Having calculated reference length "L", we need to determine the cut length value. To do this we must consider two factors:

a) tension factor

b) reduction in length due material displacement when welded

Reference length "L" is a length of the belt on two pulleys with negligible tension. In this condition, the belt does not theoretically adhere to pulleys, as there is not any tension existing withing the belt, to build enough friction to grip belt on each pulley. Therefore we have to reduce the reference length in order to increase the tension in the belt, and to create sufficient force to provide sufficient friction to hold the belt firmly on each pulley.

Polyurethane belts usually have 6% - 10% tension factor, which includes allowance for the tension decay in time. Additionally, for Unimat belts, there is small 2mm-4mm reduction in length due to material displacement when belt is butt welded. Rubber O-rings have got much higher tension factor.

The final cut length formula for the polyurethane belt can be created as follow:




CL - cut length

L - reference length

10% - tension factor

4mm - reduction in length due to material displacement when butt welded

Table of Unimat 3 and SL polyurethane 5mm diam. belt cut lengths

for PJTOOLMAKERS motor pulleys

We did this job for you. Rocket science of Unimat belts comprised in form of a simple to read table. Make your own belt like a pro for your PJTOOLMAKERS Unimat motor pulleys.






Let's put this fascinating rocket science of Unimat belts in practice. If you managed to get through the above academic tutorial, and you feel like a scholar, it is time to get some practical skills. 

List of tools and materials required to make your own drive belts:


1) 5mm diameter polyurethane belt cord

2) small tip size marker

3) ruler

4) basic soldering iron with flat, knife-like tip

5) pair of cutters

6) OPTIONAL - belt sander

belt length math form.jpg
pulley distance dimensions unim3.jpg
pulley distance dimensions unim sl.jpg
Unimat pulleys pitch.jpg
z value unimat pulleys.jpg
double groove pulley z value.jpg

Step 1. Measure required belt's length on your lathe


Step 2. Mark on the belt required belt's length with a marker


Step 3. Subtract 15mm – 20mm from the marked length of your belt


Step 4. Cut the belt


Step 5. Using hot, flat, knife-like tip of your soldering iron, melt both ends of the belt


Step 6. Immediately join two melted ends of your belt and hold them joined until it does not come apart


Step 7. Leave the joined belt for at least 30 min to completely cool down and harden


Step 8. Remove excess of polyurethane to achieve the most even joint possible. I can only suggest that a small belt sander is probably the best tool to use, but unfortunately not everyone might have at home. 

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