3d Printer Belt Tension : Is it Important?

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What does 3d printer belt tension have to do with the quality of 3d prints? Read on and find out why I checked mine and adjusted it.

The Symptoms of loose 3d printer belt tension

Over time your 3d printing quality has started to suffer from overshoot on the corners, tearing down the side and ripple along the X and Y axis.

You put it down to the printers age – or wear in the components and think – ‘I will get around to looking at that in a bit.’

Then you want to print the latest megahit, with your friends around. You download the model and slice it, cool enough. Set it up for the right material and send it to the printer.

The printer starts up and what comes out, well you may as well have got a 3d pen and done it by hand.

Update: I have added an entry into my 3d print journal where something happened to my Y axis, check out the  pictures to see what happens when the belt comes loose here.

With a sigh you think ok – ‘so I do need to sort it out’.

What first?

Well one of the main things you can do is to check your 3d printer belt tension.

Why? – if this is not right then you will have a print head that thinks for itself.

After the motor stops moving, the print head carries on, when the motor reverses the print head doesn’t for a bit.

Add this into both directions and you can see that the produced model will be a bit off.

Try printing out a test cube – I use a 1 in cube as a test print. See what it looks like. Analyse the resulting print and see if there are any problems.

How to check your 3d printer belt tension


If you have a prusa i3 or clone – like me, then you can see the belts. If you have one of the enclosed printers then you will need to read the manual to see where the belts are and if they are adjustable.

The following is carried out on a prusa and may be relevant to your model, but you can follow along to see how it’s done and translate the process to your printer.

On the X axis you have the print head carriage, travelling on its bearings. The travel is controlled by a belt secured on the carriage and going over a bearing at one side and a gear, attached to the stepper motor, at the other.

The rule of thumb is that the belt should deflect by 1/64 in per in of length, so on my prusa I have 15 inches between the center of the drive pulley and the bearing so I would expect 15/64 in ( approx ¼ in) of deflection at the center of the belt.

With the print head in it’s home position you can place a ruler upright to see the height of the belt in its resting state. You can then deflect the belt with your hand – don’t go mad or you will damage it, press down until you feel resistance. If the belt has moved more than ¼ in then it it too slack and will affect the quality of your prints.

How to correct your 3d printer belt tension

Well unless you have printed a modification, then you are left with the fixing I have. Which is the belt turned back on itself and zip tied in place.

Is this brilliant – well no, as any adjustment means cutting off the zip tie and replacing it with a fresh one.

I haven’t looked on Thingiverse or other modelling sites to see if there is a printable clamp developed for this but I will design one myself.

In the meantime, make sure you have a spare zip tie, if not then order some off of Amazon or go down to Walmart and pick up a bunch for next to nothing. You are looking for 0.07 in by 3.8in, looking around the closest is the 18lb ones – classed as miniature. So go for 4-18 size either natural or with carbon black. The code means 4 in long with 18lb tensile strength.

You should be able to pick up 100 for less than a dollar.

After you cut the zip tie off then you will need to tension the belt before adding the new one.

As I have not cut my belt very short I am able to grasp the end with a pair of pliers. Using the print head block as a lever I can put tension on the belt.

If you just try to pull on the belt the whole print head will slide across the x axis. This is not what you want.

With the pliers you need to grasp the belt and using the print head as a lever, push your hand away from the direction of the belt, with the end of the pliers tensioning the belt.

Before you start doing this you want to put the new zip tie onto the belt and start it pulling up, this way you can tension the belt with one hand and manoeuvre the zip tie into position with the other. Pulling it tight before letting go of the pliers.

Check the tension on the belt and then do the same with the Y axis belt – the length of this belt is almost the same as the X axis one so the deflection in the center will be about the same – 1/4” max.

note: you don’t want to tension the belt tighter than 6.5lbs, as the tensile loading (sideways) of a Nema 17 stepper motor is specified at this level. If you move the print head across its axis by hand you will feel if it is struggling, if it is it is too tight. I found that by pulling on the belt I could loosen it slightly, which is the reason I want to develop the belt clamp, the belts were slipping over each other.

After you have carried this operation out it is worth printing out a test piece, I use a 1 in cube. This way you can put a date on it and chuck it in a box to see how long it is before you need to tension the belts again.

Note: after fitting a new belt you will need to tighten it after a few hours use as it will settle down.

My test print after tightening the belts was flat with no overshoot, so I had been suffering with this for a long time and blaming the mechanics. Which it was but could be easily rectified.

There are methods I have seen that use the frequency of a belt tapped to check the tension, but with my machine the print head is in the way and would cause a problem with the frequency. This would give a false reading.

There are mechanical tension checkers, designed for car belts. I feel these are too heavy to operate successfully on the lighter belt on the 3d printer.

So at the moment you are stuck with the finger push and measure the deflection to tell whether it is tensioned enough.

Have you tightened your belt? Have you found a major difference between before and after, if so tell us about it. Do you have another method of checking or adjusting the 3d printer belt tension?

Many thanks for reading



One comment

  1. The belt tension situation for something like a GT2 200 seems like a tough task with a nema 17 stepper. 200mm belts don’t have a lot of span and so the balance between backlash/slack and too tight seems precarious. I’ve searched for the value of 6.5 lbs for a lateral force limit or a radial force limit for a while and have come up empty. Belt companies can give a lot of information and belt tension calculators, but for these smaller belts there is little. I doubt I’d get much frequency out of the belt at that length and even then, based on the lack of information, what is the ideal frequency for the belt and the span? If it was a timing belt for a car it seems easy. For a 3D printer, not so much. Very frustrating. I was glad to find this interesting post.

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