Removing Masthead Sheaves

[tsell]

So the challenges continue. I got the new sheaves and pins, bit when tried t put them in, the pin does not go through. Without the sheave in place the pin slides right in, but as soon as I try the new sheave, the pin no longer fits. New pins and old sheaves work. It appears as though the old holes were not drilled straight, as the mast is worn at where the old sheave wore a groove about1/32 deep in the mast head.

I think I have 2 options, either have the new sheaves machined to get more room to line up with the pin orientation, or have the old sheaves machined to accept all rope halyards. A third andI think much less desirable option would be to enlarge the holes that the pin runs through, which I think would remove some of the support.

I will try to post some pictures tomorrow.

[unclemack]

From your description it seems that the holes in the mast for the sheave-pins must not be at 90 degrees to the sheave cheeks.

If the new sheaves aren't thicker than the old ones then the obvious solution is to widen the sheave holes enough to accept the pin at that slight angle.

Definitely don't widen the sheave-pin holes in the mast.

Ideally want to keep the sheave's hole close to its present size while widening it a little at the outer edges.
With access to machine shop tools I'd use a tapered reamer from each side until the pin wiggles in the hole a little bit, then try it in the mast. A bit of sloppiness when finished is a good thing.

Redneck 2-minute method uses a power drill and a drill bit about a millimetre smaller than the hole.
Force the drill to rotate/swing around in a wide circle while holding the sheave tightly.
Works OK.

[captjcook]

It sounds as if your setup is similar to mine...one hole through the side of mast, then two through the sheave box.

Bsme not recommending opening up sheave bushing bore. I would rather see the pin bore in the mast "rattle" than that. The pin/axle will have plenty of support for application. Even if it is .015-020" loose.

Consider opening up "drilling" out just the mast hole, not the sheave box bores, it may give a better angle of attack to assemble and may correct alignment (you mentioned pin slid in freely) indicating some potential to gain alignment reducing wear on sheave cheek.

You might also have to open up attachment screw hole in pin "head" to reduce sheave/sheave box binding. Or replacing pin/axle head to accommodate.

Best of luck!


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[tsell]

Thanks for the suggestions. The pins are drilled at a slight angle to the sheave cheeks, as Unclemack surmised. It seems that I would lose some structural integrity if I drilled the mast holes larger, but I am also not keen on reaming the bushings either. Would a third option be to file the sheave cheeks to increase the clearance?

[captjcook]

You would have to turn them in a lathe and make the cheek conical...

I do not have any concerns of structural integrity if drilled out slightly. Even if you went as far as side loading the drill bit to square the bore, with the tightest drill bit you have to the pin size. Of course this needs to be done carefully. If you correct the pin head lock screw hole to hold the alignment. If the hole is .010 oversized you only have point contact anyway, it will crush/wear in a saddle, if loads are actually high enough until it gains enough support to stop, again a few thousandths. We are basically talking about farm machinery here. I would not hesitate to square up if it were my mast, not for a second.

C'mon brother and sister engineers, come beat me up!


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[unclemack]

Reason for your problems is that the design & construction of the fixture is flawed in a number of respects.

Masthead sheaves are safety-critical. Using only two self-tappers to retain each sheave pin was a very poor choice.

Welding shaped retaining plates to pins is unnecessarily expensive, requires a different size for each mast section and is guaranteed to make them difficult to remove.

I'd have used a single pin right through the mast so it could be driven out if it seized. I'd have used separate, larger cover/retaining plates with 4 fixings.
If the only option was to use individual sheave pins - drill & tap the ends for removal with a slide hammer and again use separate cover/retaining plates.

To align sheave box cheeks precisely perpendicular to a pre-drilled hole through the mast is not difficult - it's only necessary to use a pin with large push-fit spacers as an assembly jig to align and weld the cheeks to the mast.

Do the screw holes in the new pin plates even line up with those in your mast? If not the screw holes will be weakened. If they do line up I'll be surprised - but it's still a ridiculous arrangement.

In your situation I would definitely not try to correct the holes in the mast.
Difficulty of making the holes perpendicular to the sheave cheeks without some trial and error must be obvious surely?
Looser holes places more reliance on those two screws.

Filing the cheek to avoid further wear to the cheek seems - odd.

Unclear what anyone thinks the downside to reaming the sheave a few thou might be?