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It might happen that you find a few, old, different Unimat belts in your drawer, and you wonder if they can fit your Unimat machines. Probably the thirst thing you do is you try to fit them to your Unimats. But how can you be so sure that belts are not too tight or not too lose? Let's measure them! 


If your belts are round in shape, you will be looking at 5mm diameter for you Unimat 3, and  at 4-5mm for your Unimat SL. The best tool to precisely check the diameter of your belts is a Vernier gauge. Because you are a proud Unimat user, you certainly know how to read your calipers, although some modern easygoing users might prefer digital versions.


Circumferential length

Circumferential length of your polyurethane belt is almost equal to its cut length. I said "almost" as there will be a difference of 2-4mm in circumference due to some displacement of belt's material when welded.


There is a one particularly good tool to measure circumferential length of your Unimat belts. You might be surprised what it is.


Take your soft tailors tape measure and fit it inside your belt. Make sure there is no space between the tape and your belt, i.e. tailor tape fits tight inside the belt,


and read circumferential length of your belt.

For polyurethane, welded belts, true belt circumferential length (CircL) is equal to:

belt cut length (CL) reduced (-) by some displacement of material (Dm) when the belt is welded.


Amount of material displaced in welding process, reflected in circumferential length of belt, varies depending on your welding method and skills. In general we would assume a value of between 2mm to 4mm reduction in belt circumferential length as the most common one.

For black rubber O-rings, true belt circumferential length (CircL) is equal to its theoretical cut length CL.

Example 1

For the orange 5mm polyurethane belt from the attached picture, we read 280mm as its circumferential length. We need to apply simple math formula:




CL - belt cut length

CircL- circumferential length

Dm - displacement of material (polyurethane belts only) when welded 2mm-4mm



In this instance:





The true cut length should be approx. 284mm.

Example 2

For the black rubber O-ring, from the attached picture, we read 242mm as its circumferential length. O-rigs are manufactured in different, to our polyurethane welded belts, way. Because of this the read value of 242mm can be regarded as equal to the belt cut length (CL) for our calculations. 


The true theoretical cut length should be approx. 242mm

Translating circumferential length (CirL) to O-ring diameter

Having your circumferential length CircL  measured (or calculated), it is very handy to translate it into O-ring diameter because O-rings are usually specified by diameter. To do so, the following formula is to be used:

O-ring diameter = 2(CircL / 6.283)mm


CircL - circumferential length of belt

Example 3

Circumferential length of the belt is 226mm. Calculate diameter of the equivalent O-ring.


O-ring diam = 2(226/6.283)mm

O-ring diameter = 2(35.97)mm

O-ring diamter = 71.94mm

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