Metal forbearing

[Sam StonesParticipant @samstones42903]

On a warm, sticky, Saturday afternoon in Melbourne, too warm for my garage `workshop’, it passed through my mind that some of you would relish a go at the following :-

 

Being of the unqualified ilk in issues of tribology (ie. the science and engineering of interacting surfaces in relative motion), I have often pondered over the relationship between soft and hard materials. Controversially, at least for those of us who have observed certain symptoms of wear, we are led to believe (superficially), that the softer material is the more likely to wear.

 

Yet there are a myriad of examples where this appears opposite.

For example, (starting in the bathroom) – at the gum-line across my top incisors, there are grooves where toothbrush bristles have been rubbed to excess well before I learned about circular movement. Professionals of the dental kind usually advise us to use a soft toothbrush. As engineers, we know from experience that there’s trouble afoot with that concept.

In another example, this time in the workshop, holes and other surfaces in (say) steel, are generally lapped using a softer metal such as brass. I was privy to tests with a motor mower which, for some reason, showed that plastic gears survived the environmental onslaught much longer than the metal gears they meshed with.

The engine of my first mode of (powered) transport was a Cyclemaster, installed into the rear wheel of a bicycle. Unfortunately, the previous owner had failed to properly replace one of the circlips intended to hold the gudgeon pin central inside the piston. Several parts had become damaged when the circlip came adrift and buried itself into the side of the piston. What surprised me was that the old (hardened) gudgeon pin was significantly worn away where the bronze, little-end bearing made contact with the pin during use. From memory, the one-sided wear pattern on the pin was at least 0.5mm deep. Even more surprising was that a new gudgeon pin fitted perfectly into the original little-end bush made of bronze.

In other words, the softer bush showed no obvious signs of wear, yet the gudgeon needed to be replaced.

I’m fairly convinced that I know most of the answers, but only from experience, not from a tribology standpoint.

What are your views?

Regards,

Sam

Edited By Sam Stones on 04/12/2010 02:07:30

[Sam StonesParticipant @samstones42903]

Pun intended

[TerrydParticipant @terryd72465]

hi Sam,

 

I’m no tribologist either (far too specialised for me) but my understanding of this phenomenon is that the softer material becomes embedded with abrasive materials which wear away the harder one, leading to more particles etc and progressively increasing wear.

 

What’s your take on the Ashes series by the way, PM me about local reaction?

 

Terry

[John OlsenParticipant @johnolsen79199]

I think Terry is right on this. Another example of this sort of thing is that a rubber lip seal will wear a groove into a shaft.

 

regards

John

[Ian S CParticipant @iansc]

Or a friends rather elderly, and much abused  Myford 7 (don’t know what model), one day he was complaining the the back lash on the cross slide had got that bad that he woundered whether there was a thread on it at all, he said “it;ll be the nut, its just made of some muck metal”, I disagreed saying it would be the screw, as it was, the nut looked un warn. He went into town (Christchurch) to a firm that had been the agents for Myford”, one of the older staff members went out to the store room and came backwith a new nut and screw conplete, and he was charged the price it was listed at After coverting from NZ pounds to NZ dollars $NZ5 total. The lathe had been most of its life in a country motor garage. Ian S C

[Peter G. ShawParticipant @peterg-shaw75338]

Re: John Olsen’s comment:

 

For those of you who remember them, the Austin Maxi was an excellent example of this problem. In this case the rubber seal after about 3 years or so wore a groove in the primary drive gear which eventually leaked oil onto the clutch. Replacing oilseal, & clutch plate cured the problem for 12 to 18 months. Replacing the primary drive gear (at about £100 a time plus VAT) would effect a permanent cure for 3 to 4 years! I could replace oilseal & clutch plate for £15 plus my labour. Needless to say, I never changed the gear. And I had two Maxis – both were the same.

 

Peter G. Shaw 

[MikeParticipant @mike89748]

Some many years ago I wondered how relatively soft nylon fishing line could cut its way through hard chromed rod rings. The answer was that the surface of the line gets embedded with microscopic very hard grit particles from the water – the principle of lapping. I guess it’s the same with all hard and soft bearing materials when the lubricant is contaminated with even the tiniest abrasive particles..

[Sub MandrelParticipant @submandrel]

I think we are all agreeing on this one! The soft material becomes a lap.

 

My stepson told me about he new playstation game today, Gran Turismo 5 – apparently yu can choose almost any car, even a Marina! I can’t wait to rediscover the joy of fishtailing a Marina. I wonder if it has a Maxi, complete with saggy rear suspension?

 

Neil

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