Air Belt Changer not Changing, Mechanical Issue

[mflux_gamblej]

My belt changer isn't working correctly in that the air cylinders will not retract. This happens for both the high and low range cylinders. The electrical side of the system is working well. There isnt a problem with the solid state relays, my mac valves work fine. This seems to be some mechanical sticking, maybe on the shaft that holds the two idler pulley cranks. That shaft has a lot of bearings in it that are like washers, the stack is under compression up against those bearings.

At any rate, It seems crazy to me that fadal uses air cylinders that are only one way, with a very weak spring return.

has anyone replaced their air cylinders with a dual action one? (air for both directions)? any idea what size of air cylinder to buy?

This machine does not have the hydraulic system. maybe it should have that instead. anyone else with this trouble with their air cylinders?

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[vertcnc]

I have had the bolts holding the back bracket shear off for some unknown reason. Replaced socket head cap screws and all as been good. I am not sure on the Model # of the cylinders, but they appear to be BImba or American Cylinder brand. Normally the # is laser etched in cylinder tube. Very reasonable priced if you can figure out #. If not measuring would work. You would need bore, stroke, mounting style, rod diameter, any rod extension. I have a source for these if we can figure out size.

[mflux_gamblej]

Here's my measurements of the air cylinders on my VMC-20.

I bought two of these from mcmaster. McMaster-Carr

They are the same cylinder, but with air instead of spring return. I do not know why fadal didn't have an air return on these in the first place. There are a few reasons why you might not want to have air return:

- to prevent air leaks
- if there was an air leak in the cylinder you are holding away from the belts, the one holding the belt in might have low pressure and the belts could be damaged
- to save cost. the dual action cylinder is a whole $5 more or so. geeze! but you also have to plumb it in, or worse, install more watts / mac valves if you choose to handle it that way
- if the pressure is high, the returning cylinder may slam into the retracted position, and would add wear on the idler crank shaft, or other components
- on the reverse stroke, this dual action cylinder will be violent because it doesn't hit a nice springy belt.

So why would you use the air return style? So the damn thing works! Something isn't mechanically superb about my idler pulley assembly. Maybe its the bearings on the idler crank shaft? maybe the air pressure was high and the air cylinders have bent up that shaft just enough (a few thousanths would do it) so that there is slight mechanical sticktion? when these pulleys dont retract, I can just touch it with my finger and it then retracts. its ridiculously low force required to get it to go all the way home - so i am not happy at all with the spring return in this case, or its just that my idlers are just past that threshold where they wont return on their own.

I did a lot of spindle, pulley, and belt work on this machine recently as is documented on this site. In that process I had to rebuild the idler assembly to install the slightly smaller idler pulley on the top. This rebuild was done with care. at any rate, the pulleys never retracted with a snap - they always were right on the edge, and now they just don't get over the hump and back to fully retracted position.

I also cut a big job of ryton glass filled plastic recently. I cut it dry. I haven't noticed trouble elsewhere, but maybe there is some build up of that plastic? I doubt this, but it does correlate. If it got into the idler pully assemblies thrust bearings and gummed them up just enough, this might happen? The idler assembly moves very freely, and the pulleys spin very freely - so I don't get it.


My plan is to simply use Y push to connect fittings to make it so that the same air pressure that pushes one cylinder in, also pushes the other out. I hope it doesn't slam home too hard. If it does I'll run the machine at 80 psi / lowest possible. Maybe I'll have to come up with some kind of padding or spring stop? We shall see.

I also have a 4020 VMC, it has the HT hydraulic belt changer option. That thing has a hydraulic return style cylinder - same as this idea but with that hydraulic fluid, which is a bit more gentle. You can buy the HT idler option it seems from the fadal parts vendors, but you're looking at $0.9k it seems by the time you're all bought up. So I'll go ahead and try this out and let you all know how it goes. I spent $110 on two cylinders, and a pile of fittings for this and another air project of mine.

[mflux_gamblej]

here's a video showing my dual action cylinders installed. works a treat.



here's the air cylinder from mcmaster, its a bimba. I have a bag that says D-121307-3 on it. that may not be the p/n of the actual cylinder tho.
McMaster-Carr

[slowlearner]

Thanks, I will be following your lead.

[DareBee]

Use flow control valves to control cylinder speed while keeping the pressure up

[mmurray70]

mflux_gamblej, everything still working good with this conversion? Anybody else ever do it?

My high range idler is not returning all the way occasionally and giving me an error. Havent taken it apart yet to see exactly whats going on but considering swapping out cylinders like you did. Do you still have that video?

[mflux_gamblej]

mflux_gamblej, everything still working good with this conversion? Anybody else ever do it?

My high range idler is not returning all the way occasionally and giving me an error. Havent taken it apart yet to see exactly whats going on but considering swapping out cylinders like you did. Do you still have that video?

sorry, my videos are all taken down. you should be able to do the job without the video tho. i'm still running consistent with the double acting cyl. no problems.

[mmurray70]

sorry, my videos are all taken down. you should be able to do the job without the video tho. i'm still running consistent with the double acting cyl. no problems.

Ok great, so it doesnt slam out too bad? Yeah shouldnt be too complicated. Im sure ill figure it out. The cylinder mounts the same as the original on each end right?

[mflux_gamblej]

yeah it is a bolt on replacement with the exception that you need fittings for the double acting cyl (just two additional fittings). they're right angle.

you need two Y fittings too. so that the air that activates one cyl, deactivates the other range.

pretty easy.

[mmurray70]

Just a follow up, I did replace the cylinders and they are working well. They fit fine. Still fairly smooth, really not much different shifting then before. And obviously no error anymore which is great. Thanks for the tip mflux_gamblej!

If anybody else does this, when you order the cylinder from mcmaster order a 1/4" wrench thats 0.125 wide to grab the flat on the shaft as most will be too wide to fit.