belt replacement/adjusting

[Todd Mueller]

I have a relatively new 400mm by 600mm laser machine from china, brand is jinan qichen.
it is belt drive, with one belt on each side, which pulls the x-axis to the front or back of machine, and then one other belt that moves the laser head to the left and right of machine.

recently my machine had started to cut poorly and i discovered that the x-axis bar that the laser head is mounted on, was off skew, meaning when you told the machine to bring the laser head all of the way to the front of the machine, one side of the bar would get there slightly ahead of the other side, so basically this x-axis beam was not perpendicular to the 2 rails that it slid back and forth on, this of course completely messes with the laser beam and makes it cut ok in some areas of the cutting table, but very poorly in others.

I attached a basic sketch showing the layout. i also have pics showing the belt configuration, one of the toothed gears that are at the rear of the machine for the two belts, the belt tensioning device/setup, and the front rail stops where i will notice when the xaxis is all the way at the front of the machine, one side will be closer to the end than the other.

So to fix this problem this is what i did, but i works for awhile and then will get off skew again.

step 1: remove both belts from the top adjuster which sits just behind the x axis on the top side
step 2: now that both belts are removed you can easily slide the axis all the way to the front of the machine to the rail stops and zip tie it in place so it can not move or wiggle while you replace and tension the belts.
step 3: since the belts are still attached to the underside of the x axis and run towards the front of the machine, over the smooth pulley, then all the way to the back and up and over the toothed pulley then finally towards the front of the machine up to the tension mount. i have the belt over the smooth front pulley and then pull it by had towards the back of machine and wrap it up and around the toothed pulley, and then pull it by hand as tight as i can up to the tensioner on both sides and screw the holding clamp/bar down onto the belt to hold it in place. I then use the tensioners to get them relatively even tension on both sides.
step 4: now i check my laser alignment and correct if necessary, and get back to cutting and all is good....until...

eventually it shifts out of place, and one side of that x axis will be closer to the front frail stop on one side or the other when you have the x axis driven all the way to the front of the machine.

Possibilities i have thought of:
1. belt or belts could be stretched.. only a few months old and dont suspect that but it's possible i guess
2. belt could be missing a tooth somewhere and slipping, i have looked and dont see that, but it's possible
3. something catches or drags once in awhile, i have slide the x axix back and forth when the belts are off and it slides like glass, so seems the bearings in the rail glides "what are they supposed to be called?) must be ok
4. Just simply dont have accurate tension on both sides and eventually that makes it get off skew

i know the pictures may look mucky because of the lighting and the flash, but i clean and regrease the rails with white lithium grease quite often and the system is pretty clean.

[John Noell]

You show the "roller bar" as one continuous rod. However, at least on my machine, it is not. On mine, the stepper motor is in the center and the toothed pulley it drives is attached to the rods on each side by circular pieces. I do not know the names of the circular pieces that are on the rod and connect to the stepper via belt. But, if either of them is loose (they tighten to the rod with a hex head bolt), it might cause those symptoms.

[Ronald Blue]

I have no first hand knowledge here other than years of mechanical experience. Correct me if I am wrong but there is a shaft that connects the pulleys on each belt isn't there? It makes sense and appears to be validated by the last photo. I would look very closely to make sure neither pulley could possibly be slipping on the shaft. I would think there would be a keyway on the shaft and in the pulley to keep this from happening. How far is it skewed when you have to realign it? Do the belts seem tight when you check them? It's not likely they are stretching however. Is it possible the end that stays anchored on the belt could be slipping in it's mounting point? Is it always off the same direction? I hope this gives you something to think about in finding what's going on. Good luck.

[Todd Mueller]

You show the "roller bar" as one continuous rod. However, at least on my machine, it is not. On mine, the stepper motor is in the center and the toothed pulley it drives is attached to the rods on each side by circular pieces. I do not know the names of the circular pieces that are on the rod and connect to the stepper via belt. But, if either of them is loose (they tighten to the rod with a hex head bolt), it might cause those symptoms.
mine_is_a_single_shaft,stepper_on_one_end

[Todd Mueller]

great tips, i thought i had figured out if it was "getting off" like on the same side every time or not, but im not sure, i will watch it and make a note of it so i know.

someone else i had contacted, brought up that the pulleys "could slip", i have a hunch they dont have a keyway, unfortunately, but i was going to remove the screws holding one of the pulleys on and the very first one i tried, which i think actually has 2 or 3 allen screws that go in to hold it, started to strip and i just stopped and didnt try again, as of yet. if it is starting to strip what might a good approach to get it out, so that i could replace it with a good screw? any good tips?
yes there is a shaft at the rear of the machine, and that is what the two gears are attached to, but on the front they are just single individual pulleys, so no shaft, just free spinning, smooth pulleys.
it can get off by im guessing a 1/4 to 1/2" from one side to the other...very visibly noticeable.
one time one belt did seem quite a bit looser than the other side, and if and when it starts to get off quite a bit again, i will note if one belt has become noticeably looser.
i guess the belt could slip out of it's "end point" connection some, i put them down pretty tight but i could easily find a way to monitor it.

all of those are crazy wonderful tips... lots for me to consider... would love to loosen the pulleys and retighten them, just scared of stripping now that the first one acted the way it did.

*****Does anyone know if my approach to setting up the belts once they have been removed is even the correct way? I'm just winging it, no experience, just seemed like the right way to do it


I have no first hand knowledge here other than years of mechanical experience. Correct me if I am wrong but there is a shaft that connects the pulleys on each belt isn't there? It makes sense and appears to be validated by the last photo. I would look very closely to make sure neither pulley could possibly be slipping on the shaft. I would think there would be a keyway on the shaft and in the pulley to keep this from happening. How far is it skewed when you have to realign it? Do the belts seem tight when you check them? It's not likely they are stretching however. Is it possible the end that stays anchored on the belt could be slipping in it's mounting point? Is it always off the same direction? I hope this gives you something to think about in finding what's going on. Good luck.

[Scott Marquez]

With the machine in the skewed state, mark or measure the rails to record the amount of difference. Do your alignment, but swap belts from side to side and run the machine again and see if your problem moves to the other side.
One other thing to try while the belts are off is to check that the gantry moves freely, and one side isn't sticky. Your machine looks to be in pretty rusty shape for a "fairly new machine".
Scott

[Bert Kemp]

yikes I never noticed the rust and crud in there. Try giving everything a good cleaning check the belts for junk in the teeth and the pullys for rust. Wipe everything clean and then wipe down with some light machine oil. Rust and dirt on the rails will cause the problems your having.

[Bill George]

yikes I never noticed the rust and crud in there. Try giving everything a good cleaning check the belts for junk in the teeth and the pullys for rust. Wipe everything clean and then wipe down with some light machine oil. Rust and dirt on the rails will cause the problems your having.

Ditto, never seen as much rust and corrosion (from moisture? ) inside any machine I owned. I would suspect the belts have deteriorated to the point where you can never get them tight and have them hold adjustment. So where is this machine kept and what are you cutting? BTW the crud built up on the belt teeth can make it jump a tooth or so which is all it takes.

[Tony Lenkic]

I have seen something like those pictures at local distributors shop. It was a ULS laser used to cut PVC all day every day.
Person that owned it knew about potential harm but jobs were well paying and he was willing to sacrifice the machine.

[Merritt Conner]

Other then the obvious cleanliness/lubrication I would go all long the lines of Bill G's thoughts. Those timing pulleys look awfully corroded, possibly causing skipping. Possibly even so much so that new belts wouldn't even help.

[Ronald Blue]

Getting stuck set screws out is always a challenge. First you want to make sure you have good quality allen wrenches or sockets. With the corroded appearance of things it is difficult to know whether they are that tight or if they are just rusted in. One can drill as a last resort. That has it's own pitfalls. If the set screw is top quality they are tough going to drill and may even require a carbide drill bit. If they are low quality that may be why it is rounding out. I would remove the shaft and pillow block bearings to do this. Secure it in a vice to make it solid. One other thing to try before drilling is a hammer blow impact driver. They drive the bit downward while rotating to loosen. They don't always work but they are worth a shot. I would soak them thoroughly with a penetrating oil first such as PB Blaster or Areokroil. If you end up having to drill them out worst case scenario if you lose the threads is drill and tap for a size larger. If you get them out and want to be sure they don't slip you can use the tap drill and make a dimple in the shaft for the set screw to lock in to. I have done this on much larger pieces of equipment successfully many times. I hope this is helpful.

[Todd Mueller]

I have been on vacation and wrapping up my work season but am back now, i will ready these new messages/replies tomorrow and see where i am at..thank you for your help so far!

[Todd Mueller]

I did do a quick read through of the new posts, the gantry glides like glass, i have had both belts off before and i clean them often, and relubricate with white lithium grease, although yes the teeth on the belts are very dirty and mucky, i will remove them and clean thoroughly, maybe even take some measurements to make sure nothing is grossly stretched or something, which i doubt. i love the idea of when the machine gets off, take some measurements and then flop the belts to opposite sides, and see what happens the next time it gets off.

I dont know if my pulleys/gears slip or not, the next time i have the belts off, i will put some pretty good pressure on them and see if they try to slip on the rod that they are mounted too..

The one thing i dont see that anyone has replied to completely is what the exact full procedure of aligning it when putting the belts back on or putting new belts on. I stated what i do, but I have no idea if that is the right way or not, the one thing that has my suspicions up is that when i pull the rail or gantry all the way to the front of the machine and lock it down with zip ties, the tensioner is on the top side of the belts and to the rear of the gantry/rail. so when i tighten those down on both sides to make it as even as possible, it only pulls on the belt from between where the belt is mounted on the backside of the gantry to the rear of the machine where it goes around the cogged gear. the whole lower side of the belt cant move at this point because the cogged gear at the back of the machine that the belt goes around cant move, or at least i think/figured.

does the gantry just kind of self level itself from side to side somehow as long as you have the tension close on both sides? there has to be a "best procedure" for doing this, I have just never found it online or in my manual.

[Kev Williams]

Because the Y drive rod is solid, driven by a stepper on one side, then IF if the drive gear on the stepper was loose, the whole rod would be loose, and all Y travel would suffer, BUT this wouldn't cause the X rail to be skewed. Since it is skewed, it's almost a 100% certainty that the driven 'passive' gear on the other side isn't tight on the shaft.

The only other possibility as I see it would be that one of the Y rails, either the stepper OR the passive side, is so close to being totally seized up that it's easier for the belt to skip a tooth than push the rail. A seized rail could also cause even a fairly well tightened belt gear to move on the shaft...

And with all the corrosion present, a totally seized bearing would not be surprising at all...

[John Lifer]

You mentioned that you remove the belts and cleaned and relubricated with white lithium grease in the past, you didn't lubricate the belts did you?, They have to be dry and clean!

[Bill George]

You mentioned that you remove the belts and cleaned and relubricated with white lithium grease in the past, you didn't lubricate the belts did you?, They have to be dry and clean!

Plus lubing the rails? Don't they run on wheels with bearings, if so keeping the rails from corroding in this case would be my goal, otherwise lube would just attract dust. The entire machine mechanical needs to be cleaned up, I don't see how it can do any accurate work the way it is.

[Kev Williams]

Plus lubing the rails? Don't they run on wheels with bearings, if so keeping the rails from corroding in this case would be my goal, otherwise lube would just attract dust. The entire machine mechanical needs to be cleaned up, I don't see how it can do any accurate work the way it is.
They don't run on wheels, but the railguides are chock full of ball bearings that do need lube--
349899

my guess is his bearing block is loaded with chunks of rust flakes...

[John Lifer]

Was reading on my tablet and didn't look close at the pictures, but in looking at them closer, the belts are very worn. I'd bet it's jumping teeth, really doesn't look good.