Quantitative method for correctly adjusting belt tension?

I’ve read the support articles on adjusting the side & carriage belt tensions, but I’m still clueless on how to get the right amount of tension. The videos show pressing down on the belt with a finger, but it’s impossible to tell how hard they’re pressing.

Is there any more quantitative method? Easiest would be vibration frequency. My side belts, when plucked, currently vibrate at 75Hz on the free bottom half, 86Hz on the arm-attached top half (with the laser arm all the way to the back). The carriage belt is looser, around 41Hz on both halves.*

(I know I can make test cuts to see if things improve. But I suspect I have more than one problem (dirty belts, worn belts), and I’d like to eliminate tension problems from the start. And even if tension is the problem, I don’t know how to distinguish between “too loose” and “too tight” in the test cut.)

*I’m using an app to measure this, “Sonic Tools” for iOS.

1 Like

Not to my knowledge.
I notice the slot and holes on the centerline of the idler wheels, and tool marks on the aluminum there, so I speculate during assembly there is a device mounted there to adjust the belt tension. I think the simplest method would be to apply a specific torque to the “push” of the adjustment.
Speculation aside, I have found it difficult to get the belt tight enough, but haven’t managed to get it too tight by hand.

The company advises to roll it off and on, but there have been numerous cases of the plastic wheels failing, and the belts being fiber reenforced have very little stretch, so I opt to loosen the idler instead of stressing the plastic.

I think your idea of vibration frequency would work, but without a means of minute tension adjustment, may be difficult to nail down.

1 Like

Sonic measurements will vary, influenced by mic sensitivity and positioning, so unless you’re building a test rig that incorporates a placement jig for the phone mic, your measurements aren’t any more accurate than simply ‘pressing down’ on the belt.

Step 1.) Slide the moveable component, gantry or carriage plate assembly to one end of its travel path (the following test works on either belt).

Step 2.) Place your thumb and index finger on either side of both halves of the belt, approximately 2" away from the pulley wheel at the opposite end of the component travel path.

Step 3.) Apply moderate pressure to the belt, about as much as would be required to lift a half-full can of soda using the same fingers, you should be able to deflect the belt about 1/8" on either half, but not touch the belt together without increasing the amount of pressure.

Step 4.) Release the belt and move farther away, to a position 3" away from the pulley wheel, and perform the test again using the same amount of pressure. At this position, you should be able to just touch both halves of the belt together, without having to increase the amount of pressure from your fingers, if so, your belt tension is correct.

If the test does not follow this progression, adjust the tension either up or down to achieve the correct progression.


I’m trying this right now, but I don’t have much confidence in the consistency of my fingers’ slipperiness, or in my own ability to reliably replicate the same level of force. Would you say it’s about 1 pound of force?

Did this method come from your own experience, or was it advice from Glowforge support?

I’m pretty confident in the measurement, having checked it it two ways: (1) Comparing the audible sound against a musical note on the keyboard gives me the same frequency; (2) the vibration continues for several seconds, and if I interrupt the vibration with a finger, I see that exact frequency spike disappear.

In general I think that what you said is true of sound level measurements, but frequency measurements should be more stable as long as you confirm you’re measuring the right sound (not some background sound).


Thanks for the advice about loosening the idler for placing the belt, I’ll remember that.

I have found it difficult to get the belt tight enough, but haven’t managed to get it too tight by hand.

Yeah, my fingers are weak and I struggle with this. For the side belts, I used a rod to help me press against the idlers. But if the plastic wheels are prone to failure, that’s probably a bad idea.


I dunno, it took all I had in thumb strength to get it tight enough.

I developed this method because it became necessary to be able to inform people how to test belt tension without using anything other than their fingers to test. I sell replacement parts and most people don’t have any way to measure belt tension, or any reference regarding the belts as shipped from the factory.

One of the first things I did, upon receiving my Glowforge, was to disassemble the inner workings so I had a reference to guide me when things needed to be repaired or replaced. I took note of things like belt tension and wheel structure/placement/orientation during disassembly so I had a way to confirm correct reassembly, which came in handy later when I started selling spare parts.

IMHO… Glowforge has no clue when it comes to how to relate proper repair procedures to its customers.

1 Like

I see, makes sense. I’ll have my wife try it; she has more reliable fingers.

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.