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cameron
05-17-2012, 10:34 PM
I want to make some sheaves for rope blocks from acetal or Delrin. Different sizes, but for example, 2 1/4" dia.x1/2" wide. Axle pin would be 3/8" dia S.S.

Is it worthwhile using porous bronze bushes in these, or would plain holes stand up to use? Maximum working load about 800 pounds, typical loads usually much less.

Is there any advantage in using Delrin rather than generic acetal?

Dave Cameron

winchman
05-18-2012, 07:38 AM
That much load on a 3/8" axle sounds iffy for plastic. Decent ball or needle bearings are so inexpensive that it seems much better to go that way, but the bronze should work OK, too.

Peter.
05-18-2012, 03:05 PM
Don't see why it wouldn't work, they use skinny little plastic wheels on genie lifts and they have to satisfy LOLER regs in the Uk.

Mark McGrath
05-18-2012, 03:13 PM
Demag uses plastic sheaves on some pretty big mobile cranes.Usually Oilamid an oil filled nylon from Licharz.
I think they (Licharz) have something about it on their website.
It`s very nice stuff to machine.

Forestgnome
05-18-2012, 03:21 PM
Definitely use some sort of bushing if you use acetal. Under that kind of pressure the acetal can warm up. When it warms up it becomes soft, then conforms better, then creates more friction, until if finally melts onto the shaft. Had it happen to me. I ended up pressing in a bronze bushing with straight knurls on the od.

bob_s
05-18-2012, 04:43 PM
I wouldn't use an axle less than 5/8".

Mike Burch
05-18-2012, 07:17 PM
Ball and roller bearings are brilliant for shafting that's rotating at speed. They are not designed for high static loads such as found on sheaves.
Slowly-rotating things require bushes.
A high load such as you intend could be a bit dodgy on Delrin, though it's true that most marine blocks use it.
An Oilite bush would be my pick, running on an accurately-sized 316 shaft. Under no circumstances use stainless on stainless, as it will gall and lock up solid.

oldtiffie
05-18-2012, 09:25 PM
Why not buy them?

There are plenty of quite cheap "plastic" multi- and single-sheaved blocks about with galvanised (or similar) coated frames - some as single units and some made up with nylon or similar ropes. The load ratings are surprisingly good for their size.

As for replacement - they are "throw-away" items.

Duffy
05-18-2012, 10:46 PM
You know, there are some fundemental design principles that we may be losing sight of here. First these are ROPE blocks, which means that they are most probably going to be operated by SWEAT. Second, with a 3/8" pin it is again most likely that the ROPE will be no larger than 1/2' and braided at that. I seriously doubt that ANYONE will ever get those sheaves turning fast enough and/or under sufficient load that plastic deformation would be a concern. Acetal will work JUST FINE in this application.
Since you live in Metcalf, you could drop into KJP Hardwoods in Ottawa and pick up some Lignum Vitae-equivalent and make the sheaves out of wood! It worked in many Navies for a LOOONG time!:D

oldtiffie
05-19-2012, 01:03 AM
I want to make some sheaves for rope blocks from acetal or Delrin. Different sizes, but for example, 2 1/4" dia.x1/2" wide. Axle pin would be 3/8" dia S.S.

Is it worthwhile using porous bronze bushes in these, or would plain holes stand up to use? Maximum working load about 800 pounds, typical loads usually much less.

Is there any advantage in using Delrin rather than generic acetal?

Dave Cameron

With a 50% safety factor and an assumed maximum load of 800# I'd want one rated at 1,200# minimum (I'd go for 2 x 800 = 1,600#).

If you don't want a good "rope burn" wear good gloves and don't let the rope run out quickly through your hands - the more so if you try to grip it. Having your hand dragged into the block won't impress you much either.

If running out, have the rope pass over a capstan-like cylinder (warping drum) with at least two, preferably three, turns and use it to control the rope speed run-out and the rate of descent of the load.

cameron
05-19-2012, 09:20 AM
Rope 3/8" dia polyester, breaking strength approx. 2600 lbs, safe working load according to opinion. Obviously about 5200 lbs on the sheave just before the rope breaks.

Standard practice in rigging fittings is to make ultimate strength 5 to 6 times the rated working load . I'm doing barn repairs, working by myself. Since I'll be lifting things like 8"x 8" hardwood posts into place, and sliding pole rafters up onto the roof, I classify this as "over-MY-head" lifting and have no intention of whittling down the design factors.

I just want to make a light but strong "handy billy" tackle to ease the work. Carrying a couple of steel-shelled double sheaved blocks up and down ladders doesn't seem as easy as it did half a century ago.

Duffy, I have some lignum vitae on hand, but I hate to cut it up for sheaves. That stuff doesn't grow on trees, you know!

Thanks to all for advice. I'll go with the bronze bushes.

Dave Cameron

Mike Burch
05-19-2012, 06:37 PM
Dave, I'm not too strong on Canadian geography, but if you're anywhere near water, go to your local chandlery and have a look-see at the blocks they sell for yachts and dinghies.

cameron
05-19-2012, 06:53 PM
Mike,

Good suggestion, but I can't afford the blocks I'd trust.

ikdor
05-20-2012, 07:31 AM
Ball and roller bearings are brilliant for shafting that's rotating at speed. They are not designed for high static loads such as found on sheaves.
Slowly-rotating things require bushes.


I don't agree entirely. Needle roller bearings are excellent for such an application. The needle roller bearings in the universal joint of a truck drive shaft are maintenance free for 1 million kilometers.

I do agree that the delrin alone should be fine. A roller bearing will just have less friction if you can keep the moisture out.

Igor