Restoring a variable speed belt gear drive.

Flynth

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My Heckert Fw250 milling machine is in a good shape generally. However for quite a while an issue has been plaguing me. When I set table feed to some speed. It would slow down every time I switch it on until it run at its lowest speed.

It seems the variable belt drive wasn't working properly. For those that don't know how this works. There are two test of dish shaped wheels like this:
Compress_20230317_094110_0528.jpg
One set is spring loaded, the other set has a Lever. A wide belt runs between the wheels. As the distance increases the belt runs closer to the axle. As the wheels are squeezed it runs closer to the edge. This accomplishes variable speed drive.

Unfortunately the way this is done is that the movable wheel has to have some clearance between it and its shaft to be able to slide. Likewise with its key. The problem with that is that in time it basically shreds thd shaft, the key, the wheel.... Everything. This is a known fault of this machine.

I have tig brazed in lots of silicon bronze metal onto all broken parts and I've remachined them. The sliding wheel has two press fit inserts that are supposed to retain two seals on both sides. I suppose there should be some grease in there too. Seals were gone, no grease was there too. But if there is grease it will migrate out through the keway potentially spoiling the belt... So it seems like a pretty poor design.

Other than fitting a VFD, which I have no money to buy now, does anyone have any improvement ideas?

Here are few pictures of remachined keyway. They aren't great as I used a file to cut them. Also the shaft is worn too(3thou under in the middle) so I really should make a new one. One day I will, for now I'm keeping it. Any ideas?
Compress_20230317_094110_0162.jpgCompress_20230317_094110_0357.jpgCompress_20230317_094109_9842.jpg
 
My first inclination is to install some type of self-lubricating bearing at the wear interface, but I have no idea if that is practical. This is a very common type of variable speed drive, used in everything from Bridgeports to go-karts. I would look into how this unit varies from other designs.
 
since I was a maintenance mechanic I know on large floor drill presses for sure and probably bp's also that the main problems with these systems are caused from turning the motor off with variable position in high speed. then when you turn on your machine it puts force on system parts and motor trying to get to high speed immeadiately. I hated working on these after operators would not turn the variable speed to low before shutting off power(instructions say to do this).
they have some special formed plastic spacer parts that fit over the shaft and key ways, but after all the metal shafts and parts are worn they don't last without problems. best to turn variable speed to low before turning off or bear the headaches it will cause. operators and managers always said they didn't have time to do this, so I was the one who had to pay for their bad habits. all new parts are terrible expensive but needed to work good again.
 
since I was a maintenance mechanic I know on large floor drill presses for sure and probably bp's also that the main problems with these systems are caused from turning the motor off with variable position in high speed. then when you turn on your machine it puts force on system parts and motor trying to get to high speed immeadiately. I hated working on these after operators would not turn the variable speed to low before shutting off power(instructions say to do this).
they have some special formed plastic spacer parts that fit over the shaft and key ways, but after all the metal shafts and parts are worn they don't last without problems. best to turn variable speed to low before turning off or bear the headaches it will cause. operators and managers always said they didn't have time to do this, so I was the one who had to pay for their bad habits. all new parts are terrible expensive but needed to work good again.

This tip should be preached far and wide! They should sell stickers to put on all variable speed machines that say “Turn the variable speed to low before shutting off power”.
 
My first inclination is to install some type of self-lubricating bearing at the wear interface, but I have no idea if that is practical. This is a very common type of variable speed drive, used in everything from Bridgeports to go-karts. I would look into how this unit varies from other designs.
It did cross my mind too. Perhaps one could be machined from a Teflon rod.

I did search for information on other aplications of this drive, but it seems it is always drawn with no details whatsoever, but I found some bridgeport disassembly videos, likewise for a combine harvester with such drive. It appears those shown all just have a bushing press fit in the pulley that slides on the shaft. Leave it to Germans :) to overcomplicated the design with ball bearings, multiple seals, very fancy micro-milled keys with ball bearing passages... Then their super complex design fails the same. Oh, well...

I also found a maker mark, so it seems the factory that made the machine didn't make it as it has remains of "Eberhard Zeng Ko" sticker. Perhaps someone heard of this maker?

There is also an alternative solution I didn't expect.

Specifically there are 4 wheels (2 sets). On the good side the moving wheel is spring loaded, the "stationary" just sits there at the back of the shaft.

Now, the best part. After careful disassembly of the "good" side I believe all 4 wheels started as exactly the same. Even "stationary" wheels had the ball bearings inside and can in fact move along the shaft. Later someone modified/lost them on the bad side of the gearbox. This means that I essentially have a spare wheel on the good side. So I could simply use it on the other side if the shaft was in good condition. Sadly it isn't that great so I keep this option open for future.

Regarding grease migrating away. I discovered in the good side there are supposed to be seals too. I managed to get o-rings that should work as those seals.

So I decided to make do with the currently, slightly undersized shaft, to set the stationary wheel with its key as a light press fit. I bought those 3mm ball bearings and I made the key in a shape that retains the grease and balls.

Balls are in a ring around the shaft on both sides of the wheel, and they are in the keyway. The "good" key is relieved on both sides enough so balls can be present along its mating surface with the shaft keway. So the key rides in its way on ball bearings.

However, not everything is clear to me. The problem I now have is that I imagined the balls would roll and work as in THK linear bearings for example, but there is no way those balls can recirculate in this design! There is a passage between balls in the keway and on the shaft circumference, but when moving the wheel up or down the shaft both sides of the keway move in the same direction. There is no "return path" for the balls as is in THK linear bearings. So either the balls don't roll and just slide, which is quite peculiar. If they slide, why not do away with them like 99% of the rest of those gearboxes are made? Or there should be enough free space there for the balls to be pushed from the front to the back and the other way when moved along the shaft. If this is the case then there is another issue. If the balls are supposed to go from the keyway to the circumference around the shaft they would be making a 90 degree turn where the keway ends. There is no way those balls wouldn't jam in that corner unless there was some sort of curve to guide them, but there is very little key material to shape the bottom part of that curve.

This is how it could look from the side:
Cyan - grease seal
Yellow - the key
Blue - keyway surface
Purple - the wheel
Red - ball bearings
Compress_20230318_012820_0453.jpg
Without that yellow "ramp" to guide balls up I doubt they wouldn't just jam in the corner, but such a tiny piece of material is going to wear in no time... But I'll likely try to add it to my new key. That would be the fourth attempt of making that key. The first didn't block the grease, the second didn't have any provision for balls, the third has no "ramp", but a 90 degree angle in the place shown in the drawing.

since I was a maintenance mechanic I know on large floor drill presses for sure and probably bp's also that the main problems with these systems are caused from turning the motor off with variable position in high speed. then when you turn on your machine it puts force on system parts and motor trying to get to high speed immeadiately. I hated working on these after operators would not turn the variable speed to low before shutting off power(instructions say to do this).
they have some special formed plastic spacer parts that fit over the shaft and key ways, but after all the metal shafts and parts are worn they don't last without problems. best to turn variable speed to low before turning off or bear the headaches it will cause. operators and managers always said they didn't have time to do this, so I was the one who had to pay for their bad habits. all new parts are terrible expensive but needed to work good again.
When this gearbox operates the spindle this is no doubt true, but my gearbox powers table feed. There is no way to "leave it on lowest speed" and wind it up from low to high on every single start. This milling machine is supposed to engage its table feed immediately at correct speed. The manual specifies the feed settings should be made while it operates, but it doesn't mention leaving it at any particular setting.

Also, this machine has two additional features that would make it impossible to use. It has sort of rudimentary "programming" with variously shaped plastic "bricks". You attach those bricks to the table and there is a series of switches that pick them up as table moves. By setting correct bricks at correct positions you can have the feed stop somewhere, engage fast return, feed the other way etc. The manual makes a big deal out of using this feature with the preset feed rates.

This tip should be preached far and wide! They should sell stickers to put on all variable speed machines that say “Turn the variable speed to low before shutting off power”.

I bet his is applicable only to spindle drives. I can't imagine winding a table feed up and down every single time it's engaged.

Perhaps this is the reason for this ball bearing design? They tried to improve the sliding design?

Actually, now that I think about it, there is a much larger variable speed drive for the main spindle. Other than to wind it up and down while it operates the manual also doesn't mention leaving it at any particular setting. Personally I've have it on one setting (around the middle) for most of the time. I think winding it up and down every single time wouod wear it in no time. However, I think this must be a different design than ones you're talking about. It doesn't contradict your experience of course.
 
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Here are some pictures of the Worthington 3B AllSpeed drive that powers the spindle on my Sheldon MW-56-P lathe. It uses the same concept as the table drive in your milling machine. Note on this drive only 1/2 of each sheave moves. On the moveable half it has a bushing between the shaft and the body of the sheave. It also has a set of pins between the fixed half and the sliding half to keep the rotating halves in sync.

One major difference between your machine and the AllSpeed drive in the Sheldon lathe is that opening and closing the sheaves on the Sheldon is done by a gear motor rather than manually.

The assembly rides on a set of bearings that can be lubricated as well as having lubrication points on the moving halves of the sheaves. The bearings and bushings do require some grease, but minimal amounts at fairly wide intervals. There is a plate fastened to the base of the assembly that cautions "To insure best operation shift through entire speed range at least once a day. Grease once each 400 hours of operation with light cup grease, using a handgun. Caution-excessive pressure will blow out bushings."

It's possible your machine did have bearing seals at one time that were destroyed by over greasing the bearings.
 

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Here are some pictures of the Worthington 3B AllSpeed drive that powers the spindle on my Sheldon MW-56-P lathe. It uses the same concept as the table drive in your milling machine. Note on this drive only 1/2 of each sheave moves. On the moveable half it has a bushing between the shaft and the body of the sheave. It also has a set of pins between the fixed half and the sliding half to keep the rotating halves in sync.

One major difference between your machine and the AllSpeed drive in the Sheldon lathe is that opening and closing the sheaves on the Sheldon is done by a gear motor rather than manually.

The assembly rides on a set of bearings that can be lubricated as well as having lubrication points on the moving halves of the sheaves. The bearings and bushings do require some grease, but minimal amounts at fairly wide intervals. There is a plate fastened to the base of the assembly that cautions "To insure best operation shift through entire speed range at least once a day. Grease once each 400 hours of operation with light cup grease, using a handgun. Caution-excessive pressure will blow out bushings."

It's possible your machine did have bearing seals at one time that were destroyed by over greasing the bearings.

Wow, thank you, this is really useful. Especially the text on the plaque. The manual for my machine is very sparse. Lots of stuff is not mentioned. For example regarding gib adjustment all they tell you is gib screw locations... So it is very likely winding it up and down every so often is a good thing to do for my machine too.

Photos are very interesting too.

Since my last post I've disassembled one piece on the good side and on the side of the keway the key looks like this. I made a mistake before, the balls don't go up into the channel around the circumference. They are retained in the key

This is looking from inside the shaft towards the key:
Compress_20230318_130333_3617.jpg
And the some photos with drawing added. Notice someone even added rollers in there... But they are sideways to the motion! So I have no idea why. Maybe just to take up space? Still I'm recreating the same key. Yes there are seals too.
Compress_20230318_131238_8521.jpg
Compress_20230318_131141_1204.jpg

If anyone has or has seen another variable speed drive with ball bearings used for moving up and down the shaft rather than for rotation please let me know.

Also I wonder what you all think. Is this indeed an original design? Did someone mess with it? One clue the balls are original is that many years ago when I bought the machine I spoke with a guy from a company that specialises in rebuilding those heckert machines. First thing he asked me when he heard I bought one was "is your variable speed drive messed up"? When I said its not that bad he warned me "not to loose the little balls" when I disassemble it. So some balls were there from the start for sure. His company was also selling reconditioned drives. I wish I asked him for some photos, but this being their business I doubt they would want to share too much.

Edit: I made it like this:

Note the mill scale is there because the only stock I had was pretty much e act size needed. Also I'll need to redo it properly sometime soon. With a new hardened shaft and key. With nicely cut key slots in the wheels. So this is just for now.

Compress_20230318_143743_3037.jpgCompress_20230318_143743_3163.jpgCompress_20230318_143742_2694.jpg
 
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An update.

It seems to be working fine :)

However, the shaft is keyed and sits over another shaft that goes to the motor. As the motor shaft is shorter the only way to retain it is a m8 screw. I've been resorting to screwing it in very tight (with an electric impact) wrench, but still it works its way out sometimes.

I tried blue locktite, same thing.

So I wonder if I'll have to figure out some better method by drilling over the head and installing a wire (but then what to attach it to?).

So my question is, does anyone know of a method to prevent a screw from backing out in this arrangement? (blue locktite didn't work) :
Compress_20230319_084110_0117.jpg

 
This is a very common type of variable speed drive, used in everything from Bridgeports to go-karts. I would look into how this unit varies from other designs.

Sorry for the thread diversion, but CVT car transmissions use this same technology. In cars, the belt is made from loosely joined plates of metal, that only operate in compression. If one of the sheaves gets out of time, allowing the belt to go limp, the entire belt assembly can blow up and scatter link plates throughout the unit, wrecking havoc!
 
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