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Metal bar sizes

Forum for discussion relating to the Sherman.
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Adrian Harris
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Metal bar sizes

Post by Adrian Harris » Thu Apr 17, 2008 8:23 pm

I've stripped my Sherman down to the bare essentials and decided to re-investigate replacing the bushes in the wheels with bearings.

I've bought a couple of roller bearings with oil seals which look ideal but when I tried them out the axle is quite a sloppy fit.

The bearing inside diameter is 8mm but the axle is only 7.85mm.

I've measured some spare 8mm bar which I purchased to use as practice material and this too is around 7.8mm in diameter. Is this usual for bar stock or is it an imperial piece sold to the nearest metric size ?

The other question is how sloppy should the bearing fit be ?

I would have expected it to be quite a close fit. I can visualise the axle bumping from one roller to the next as the wheel rotates, rather than being supported all the way around and rotating smoothly.

Adrian.

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Armortek
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Post by Armortek » Fri Apr 18, 2008 8:36 am

Adrian

When we designed the Sherman we decided against roller bearings because we did not feel that it would be possible to keep the dirt out, and thus get a reasonable bearing life, with so little room available.

On the question of bar sizes, the imperial equivalent is 5/16", which is 7.93mm. Stock bright bar varies in size considerably, plus or minus 0.2mm is not unusual. Your best option, if you wish to run roller bearings on the shaft, is to use silver steel. This is accurately sized and can be purchased in 300mm lengths. You should note that if it is not hardened, you can expect very rapid wear, when used with a roller bearing direct onto the shaft. Another alternative is to use dry fit plastic bushes. These are easy to fit and give low friction and extended life.

Mark
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Allan Richards
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Post by Allan Richards » Fri Apr 18, 2008 8:37 am

Are you sure its 8mm bar that you have and not an odd imperial size. You can by ground silversteel stock which is accurately ground to size. I would also say that ideally the bearings shoud be a good slide fit on the axle but a light push in the aluminium wheels.
Allan Richards

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leesellars
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Post by leesellars » Fri Apr 18, 2008 8:39 am

Hello Adrian

Bearings are a subject with debait. The Bearings that are on the Tanks (Ball) and roller are meant for fast rotation and high loads.With us tank builders the wheels rotate slowly and therefor they tend to seize up quickly as miosture and dirt builds up and therfore getting a failure of the bearing.As with my Sherman has been running for years now and the wheels are fine.The Bushes are designed for slow rotaton and are ideal for it. Just grease them up. :D

Lee
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leesellars
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Post by leesellars » Fri Apr 18, 2008 8:41 am

Hello Chaps

That must be a record with 3 postings all at the same time. :lol: :lol:

Lee
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Listen carefully i shall say this only once.
If its not broke dont fix it.
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Adrian Harris
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Post by Adrian Harris » Fri Apr 18, 2008 9:46 am

Lee, doesn't look like anyone's doing any work this morning (with the possible exception of Mark, as it is his forum :wink: )

To answer the replies:

Mark:

Adrian

> we did not feel that it would be possible to keep the dirt out,

These bearings are a drop-in replacement for the bushes, but with rubber oil seals at each end, supposedly to prevent the ingress of dirt/moisture.

> Stock bright bar varies in size considerably, plus or minus 0.2mm

So it looks like it maybe difficult to locate an accurate enough length. :cry:
I suppose the other alternative is to buy 10mm and turn it down to an accurate 8mm.

> You should note that if it is not hardened,

It's years since I did o-level metalwork but I seem to recall this requires heating then quenching ? Would it also need to be tempered to withstand shocks or just the hardening ?

> Another alternative is to use dry fit plastic bushes.

Would PTFE be suitable ? I happen to have some bar stock of about the right size.

The wonderful thing with these models is that it is so easy to make changes like this :lol:

Allan:

> Are you sure its 8mm bar that you have and not an odd imperial size.

No. It was bought as 8mm but as Mark has said, what I have may be within acceptable (!) tolerance for 8mm.

> As with my Sherman has been running for years now and the wheels are fine.

Have you stripped down the wheels recently ? I'd be interested to know what level of wear you have on the axles as

> The Bushes are designed for slow rotaton and are ideal for it. Just grease them up.

As I have two Shermans, once I get both built it will be interesting to have one with bushes and the other with bearings and then compare both the performance and wear.

Adrian.

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