Filling a Zamak/zinc alloy step pulley cone with epoxy for stress crack prevention?

[TimNJ]

Hi all,

After about 9 months of slow (but steady progress), I am finally ready to re-assembly my refurbished Walker Turner 900 series drill press. Generally speaking, the build quality of the machine is excellent and have no doubts about durability about most of the iron/steel parts.

Then comes the cast zinc pullies.

The motor pulley could basically be replaced with an off-the-shelf pulley if something was to fail, so that one does not worry me much. On the other hand, there is the the spindle pulley with two integrated extended race bearings (unobtanium except through Walker Turner Serviced Machinery, LLC), and integrated bronze splined bushing.

My pulley (not unlike other's) developed some hairline stress cracks along some of the casting lines, especially where there is no fillet provided to limit stress. To (hopefully) alleviate future splintering, I decided to add some aluminum filled epoxy in the cracking areas, then ground to form a more natural fillet in these areas. Maybe this will work - or maybe the epoxy will crumble off in a few years - I have no idea.

My question is: Is there any possible down-side to filling up the pulley completely with some sort of resin or adhesive? Maybe a urethane or polyurethane? Essentially, the pulley would become one solid piece. As I understand it, pulleys are cast as they are to save on material, not for functional/performance reasons.

I also believe in "if it ain't broke, don't fix it"...but...before I put the thing back in forever.

I also drew up a rough drawing of the existing pulley if I ever decided I needed to machine a new one out of aluminum/cast iron. Just for reference.

Thank you,
Tim

 

[lucky7]

I have the same drill press, have used it for over forty years and like it. You may have balance issues at higher rpm with your plan. If it were me, I’d just use it as is.

 

[TimNJ]

Thank you for your advice. I thought of that; but wondered “how off balance could it be if the same volume of resin was poured into each section?”. Somewhat rhetorical question - I suppose it doesn’t take much imprecision in the pour to throw it off. Then again, my epoxy “fix” is not particularly well balanced either.

 

[dalmatiangirl61]

Pack it full of JB or Devcon, static balance is easy enough, and just run it. I tried finding a new off-the-shelf replacement for mine, found a few close, but not close enough.

 

[BOB-OO]

Easy part to make out of 6061, I'd make a drawing 1st, fill it w Epoxy, you could do a static balance if you really wanted to (I wouldn't bother) I'd put it back together and let her eat. If you pick up any vibration then balance it.

 

[Scottl]

Instead of going through all the hassle of reinstalling it before finding balance issues, why not mount it in your lathe on a temporary arbor first and spin it at various RPMs to see if it is out of balance. Then you could add more epoxy/sand off epoxy as needed for better balance.

 

[TimNJ]

Pack it full of JB or Devcon, static balance is easy enough, and just run it. I tried finding a new off-the-shelf replacement for mine, found a few close, but not close enough.

Thank you. I’ve never tried to balance a part like this. My initial intuition is to hang it from string and check for level on X-Y plane…by eye I guess? (There has to be a better way?)

Can get a piece of rod precisely the size of the bore, and then drill a very accurate hole through the center. Add a “shoulder” to the rod (with a washer, etc.) so that the the whole pulley can be suspended by the string without rod falling through. Hopefully, in this way, pulley hangs through actual rotational axis. Just some ideas.

 

[TimNJ]

Easy part to make out of 6061, I'd make a drawing 1st, fill it w Epoxy, you could do a static balance if you really wanted to (I wouldn't bother) I'd put it back together and let her eat. If you pick up any vibration then balance it.

Thank you. Just to clear my confusion - you are suggesting to make a new pully out of 6061, and then fill it? Or are those two separate thoughts?

 

[TimNJ]

Instead of going through all the hassle of reinstalling it before finding balance issues, why not mount it in your lathe on a temporary arbor first and spin it at various RPMs to see if it is out of balance. Then you could add more epoxy/sand off epoxy as needed for better balance.

Thank you. A good idea, but I have no lathe at the moment. My idea to make a new pulley was a possible future project. The only other issue is that the bearings are integral to the pulley assembly so it will not spin directly with an arbor, unless I misunderstood your idea.

A quick search seems to imply that static balancing is probably sufficient for the angular velocity of these types of pulleys. About 600rpm @ r = 2.5” and about 5000rpm @ 0.5”, if memory serves me correct.

 

[dalmatiangirl61]

Don't over-think things, KISS (keep it simple, stupid)

 

[TimNJ]

Don't over-think things, KISS (keep it simple, stupid)

Ah, yes that is much better, no need to trust the ol' eye ball to figure out if it's indeed balanced. Thanks!

 

[TimNJ]

Not that anyone is interested in this, but just in case. Wound up filling the whole thing with MetalSet A4. Made a little tool to level it out and to make the fillets nice.

MetalSet A4, McMaster PN 7500A11. $23 for a set of 11oz tubes. In my experience, everything from Loctite, 3M, or Devcon runs so much more expensive per ounce. JB is pretty cheap, though.

Well, hopefully it'll stand up to the vibration OK and not make more trouble instead of helping. If I don't reply to this, then you'll know it's holding up alright. It's now installed in the machine (for the first time since I took it all apart last summer), and is working well. No problems with vibration/balance that I can tell...yet.