12 replies to this topic

[Dusky]

Hi!

I wonder, what's the point of a primary gear bush that's floating?
So far I've only seen the front one being floating. But the rear will still need machining ( haven't found a floating rear bush) so what's exactly the point of having a floating front bush?
Been looking into this as I've got a gauged primary gear ( only the rear bush ofcourse..) and a budget too small to pay the local (expensive!) Machining costs

Cheers

[Spider]

I'm genuinely interested in what others may have to say, but as I se it, that end of the Primary Gear, with the actual gear on it, is loaded more than the other end, so it does need a bit more support (and to put it in very simple terms, a harder bush). The factory did this with a steel backed bush.

 

The floating bush to my way of thinking and having examined this in some detail is offered as a cheap and lazy solution to a problem that is a bit more complex than what these bushes can do.

 

Probably not a proper answer, but the material used in them is nothing new, been around since long before the Mini, and one that the factory may have looked at I'm sure, yet they chose a few times when the Primary Gear was reviewed not to go this way.

 

<EDIT: Another possible reason for these bushes coming in to being is that in the UK, there was no factory information on the correct replacement procedure for the originals, so goodness knows what inadvertent bodges this lead to and so the originals - unfairly in my books - got a bad reputation. The correct procedure was included in the Australian Workshop Manuals though. The originals were very good and didn't damage the crankshafts. I've replace many over the years using the factory advice and had good life from them.>

 

<2nd Edit:  The original bushes were 40 HB in hardness, while the floating bushes has a measured hardness of 110 HB and the crankshafts are 180 - 230 HB. No wonder they eat cranks! 
The Back (Top Hat) Bronze Bushes I note are extruded and while  haven't yet tested them for hardness, would be work hardened from the extrusion process, so even harder again than the floating bushes. >

Edited by Moke Spider, 17 April 2017 - 09:12 AM.

[Dusky]

I've been reading a bit about them, most of my info comming from your posts ;)
It's weard/sad that they even sell things like this. Think Montpellier mini man on here had problems with them too recently.
The correct material is DEVA if I recall right?

[Spider]

The original (since 1963) rear bush is actually DEVA material, yes. The front Bush is a lead bronze; laid on to a steel backer.

 

The Deva bushes are NLA and as there's little interest in any of the main stream suppliers in them, I'm in the process of having a small batch made for my own stocks.

[ACDodd]

These were developed to overcome the tendency of the stock bearing spinning in straight cut gears. They work well and have never had. Problem with one. I only use these in hotter builds, all my road stuff uses the stock bearings.

Ac

Edited by ACDodd, 17 April 2017 - 12:36 PM.

[Allrounder]

Just wondering what is the recommended running cleanace for the primary gear bush or what has been the most sucks successful clearance over the crank. Is it the same for large and small bore engines??

[Spider]

Another engineering aspect of the floating bush is that it forms what's know a rolling bearing element. That is because instead of sliding over the shaft as a bush will, it rolls between the shaft and the primary gear. It does also slide in this type of set up, but also rolls. Bushes are referred to as a type of Plain Bearing.

 

Here's some general comments on rolling and plain bearings;-

 

"If ideal lubrication and operating conditions (such as load ranges, speed ranges, temperature ranges) are maintained, plain bearings will not fail."

 

"Even if proper lubrication and operating conditions are scrupulously maintained, rolling element bearings will eventually fail by fatigue because of subsurface voids, cracks or inclusions."

 

http://machinerylubr...-bearings-gears

 

We know this to be the case as, for one example, we replace wheel bearings every other week (so it seems), yet the bushes in our trailing arms are replaced maybe every 20 - 30 years.

 

 

Just wondering what is the recommended running cleanace for the primary gear bush or what has been the most sucks successful clearance over the crank. Is it the same for large and small bore engines??

 

The Factory Clearances for the OEM Bushes are the same for both types - 0.003 to 0.0045" however, the current range of non-genuine bushes are something else and best speak with those who make / supply them.

[Dusky]

It really seems like a weard solution that isn't a solution for what they advertise it ( " no need to pay a machinist to ream the bush" as you still need to replace the rear bush too).
There must be some other way to improve/, make the gear more easily serviceable. But how, that's the question. Think I'll put my thinking cap on ^^

[tiger99]

I agree. It is not a solution at all and only exists because a particular parts supplier could not be bothered to source proper DEVA bushes, which, as we all know,last a very long time.

Google for DEVA and you will find that actually there is no problem sourcing it and anyone with a reasonable automatic lathe, not even full NC, could turn them out in modest size batches.

There is a modest business opportunity up for grabs here...

[Spider]

It really seems like a weard solution that isn't a solution for what they advertise it ( " no need to pay a machinist to ream the bush" as you still need to replace the rear bush too).
There must be some other way to improve/, make the gear more easily serviceable. But how, that's the question. Think I'll put my thinking cap on ^^

 

The proper bushes last a reasonable period of time but yes, i agree, that they could be improved on. It's not an ideal set up, but more or less, what I see is what we're stuck with.

 

I do agree with your comment and Tiger's that these modern bushes are a 'solution' to a problem that just doesn't exist.

 

Keen to hear and see any alternative ideas you may have on this.

 

A good friend of mine once did a needle roller conversion. It was rather extreme and really, an awful lot of work as well as over kill for this application. i actually felt it a backward step as in a static state. (eg when the clutch is engaged) the load rating of the bearings is less than that of the bush. He never got around to trying it as he lost his house and shed in a terrible fire about 18 months ago.

[GraemeC]

I agree. It is not a solution at all and only exists because a particular parts supplier could not be bothered to source proper DEVA bushes, which, as we all know,last a very long time.

Google for DEVA and you will find that actually there is no problem sourcing it and anyone with a reasonable automatic lathe, not even full NC, could turn them out in modest size batches.

There is a modest business opportunity up for grabs here...

 

No, as already stated that bush is lead bronze on steel so nothing to do with 'can't be bothered' as DEVA would still be the incorrect material.

 

But then that wouldn't fit with your relentless, tiring supplier bashing (the same supplier(s) that although not perfect keep these cars , ad hence hobby, of ours alive).

[Sprocket]

The floating bush will rotate at nearly half speed of the rotating parts, those being the crankshaft and the gear its self, if propperly installed.

What you have to consider with any bush like this is how the bush is lubricated. It is relying on simple splash lubrication, as such is minimal. A bush running at a full speed of the other rotating parts, is more likely to break through that minmal lubrication film than it would if the bush is running at half the speed of the other rotating parts

[tiger99]

GraemeC, the correct bearing material is, despite what you may think, DEVA, as anyone familiar with the history of the Mini would know..