[nick mulder]

Hello,

So I have a timing belt cog (under tension) with a 13.4mm bore that is fitted to a shaft with a 12.9 OD... With only 15mm or so of surface bearing thats quite a wiggle those dimensions will allow. What has happened is that the key in there (which appears slightly shallower in dimension that the original slot in the cog would indicate should have been) has been eating away at the key slot on the cog as pictured in the diagram...

Heaps of slop combined with the wiggle = noise and not a sustainable situation.

It does turn both ways, but one way much more than the other - the way that it has worn the cog obviously - the key and the shaft are fine ...

What to do ? its half a mm of slop, which means at least 0.2mm more metal required in the cog or the shaft to fill the void

What is the right approach in this situation ?

I'd like to keep them concentric obviously:wave:

Any help appreciated

[nick mulder]

shaft under load:



not concentric ...

[HuFlungDung]

If the shaft is good, then replace the timing belt pulley.

If the pulley has enough material, you can machine it out and press in a steel hub with a thick enough wall to contain the keyway.

If you really want more punishment, you can bore the pulley out until it is round and straight, but go no farther than just barely cleaning it up. If this still permits a sleeve with a .5mm wall thickness, then proceed to cut a new keyway in another position. Make a thin sleeve to adapt the new bore to the shaft. Then mill the keyway slot through the bushing (essentially splitting the bushing). You might want to cut an extra deep keyway in the pulley and use a taller than standard key. Assemble the pieces, perhaps making judicious use of a Loctite product.

[Boltz]

JB Weld has better gap filling ability than loctite. You could slip little pieces of shim in while spinning it with an indicator to get it to run true.

If it's a high stress joint that's mission critical, you'll want to change to a heat shrink interference fit.

-Jim Hart

[nick mulder]

The shaft is an input to a lathe gearbox the belt goes to the motor ... I would have thought it should be quite tight/concentric just to reduce further wear, but its not like at the business end of a machine where runout is a concern. I think after havng seen it run, wobbles and all that maybe just a EBI 'eye ball indicator' will be enough

So maybe boring a hole in something (alu ?) thats a nice push fit for the shaft, but without the key - loctite it on there then very lightly turn it until its a nice push fit into the pulley - then of course also mill out the section where the key goes ...

I might file out the chewed keyway on the pulley until its the right shape again so they key has a nice normal face to bear upon then fill in the key slop with another small peice of suitably ground/milled/fitted metal, maybe just glue that into the pulley.

I dont really want to modify the shaft destructively at all, its elaborate enough at the other end for me to be scared of ballsing up an attempt at milling in a larger keyway slot

Sound reasonable ? Any improvements to this process ?

Shim stock I found very hard to coil and fit in there without concentricity issues

[Kiwi]

Make into taper-lock setup.
Bore a tapered hole in the pulley, make tapered cone with split. Clamp together with plate and cap screws.

[nick mulder]

Yup,

Hmmm, thing is - its on my lathe - I only have a spare jewellers lathe without a compound rest so taper is out for the time being...

What do you think of this:

:: MOGLICE ::

specifically the 'DWH 310 FL' product - its like metal filler in a can which can be machined easily (with a micro lathe)

[handlewanker]

Hi, the shaft and key look good, but the pulley is beyond repair.

Get a new pulley and bore it on the lathe and fit to shaft....simple....key way in new pulley can be cut with a boring bar and tool on it's side by racking the saddle in and out, done that many times.

Any attempt to fix the existing pulley is a compromise, unless you bore and bush it, or bore for a taper lock on another lathe.

A new pulley is not that expensive in alluminium surely?

BTW, the pulley bore length needs to be at least twice the bore diam to prevent the same thing happening again.

It would appear that the key length in the bore does not have enough wall length to carry the side load of the drive, hence the hammering that eventualy destroyed the pulley bore.
Ian.

[nick mulder]

Hi, the shaft and key look good, but the pulley is beyond repair.

Get a new pulley and bore it on the lathe and fit to shaft....simple....key way in new pulley can be cut with a boring bar and tool on it's side by racking the saddle in and out, done that many times.

Any attempt to fix the existing pulley is a compromise, unless you bore and bush it, or bore for a taper lock on another lathe.

A new pulley is not that expensive in alluminium surely?

BTW, the pulley bore length needs to be at least twice the bore diam to prevent the same thing happening again.

It would appear that the key length in the bore does not have enough wall length to carry the side load of the drive, hence the hammering that eventualy destroyed the pulley bore.
Ian.

Pulley has been repaired - I bored it out to a larger size on another lathe, then made shim with both the correct ID for the shaft and OD for the new pulley, slotted out the area for a key then made a new key.

The bore was to remove the damaged area and to allow the shim to be made more easily (thicker), I didn't go so far as to make the new key an unsustainable dimension, new keyway was carefully done with a jigsaw (non-orbital) using the remaining old (undamaged area) keyway slot as guide.

A new pulley would be easy to make if I had the right gear, only slowly catching up - finishing off a new XYZA CNC mill which will be ideal...

[handlewanker]

Congrats on your quickfix, all roads lead to Rome eventually.

That reminds me of a similar situation I had.....still have.
Some years ago I bought a 1/2" bench drilling machine that was described as a "semi pedestal drill", due to it having an extra long length to the pillar, which made it necessary to make a low table 600mm high to get the table and head up to normal operating height.

Long story short, I wondered why it had a bit of vibration when in the middle pulley range and supposed the motor multistep pulley to be unbalanced, so I pulled the pulley off.......wrong pulley, wrong motor......I bought it from a deceased estate, and the previoud owner had decided to use the motor which was 3/4 hp single phase 1420 rpm for another project and "refitted" another motor of 1/2 HP capacity to the drill....grrrrr.

The plot thickens, the new motor had a shaft about 5/8" diam (15.875mm) whereas the pulley had a bore 16mm diam, a difference of .125mm (.0049").

No problem, the previous owner had wrapped a piece of brass shim round the motor shaft and pushed it back into the pulley.....but the shim was loose and the grubscrew in the keyway just displaced the pulley off centre a few thou, enough to make it wobble, and when I get a round tooit, I'll bore it out and refit it.......but, I'm about to sell it off as I have a vertical mill and another small 5 speed bench drill so who needs that many drilling machines.

BTW, I expect the pulley you reworked was a special to work with a toothed belt, so no off the shelf available.

This sounds like the lathe you have is of Chinese origin.....why anyone would fit toothed belts, with their need to be really tightly tensioned, as opposed to a single or double Vee belt if extra torque capacity needed, beats me.

I drive 3 hp at 3,000 rpm through two A section vee belts on my old capstan lathe with no dramas......motor pulley is 3" diam.
Ian.