A long time ago I did a cutdown on a used chain tensioner to see 'the go of it'; but as so often, I never got round to writing anything up.

I think both tensioners pictured are part number 03E109507S, the latest version, which has been around since Aug 29th 2005 according to oemwolf.com

 

So here are some pictures of a new unused, and some of that cutdown, annotated with all the information I could glean by just looking at it.  It has many features in common with a hydraulic lifter/tappet (an oil chamber with a one-way valve so that when under compression it resists, but when able to it refills with oil if there is oil pressure available).

There is no positive sealing between piston and its bore, and there's a hole in the business end of the piston that could let air or oil through in some conditions.  At first glance this hole is sealed both from within by a plastic plunger whose curved end meets a shoulder within the piston ahead of that hole; and from outside because it bears against the back of the nylon guide rail (with PTFE running surface) that actually tensions the slack side of the chain.

 

Let's start with a shot of the tensioner in place from an internet pic cropped from an article at pmmonline.co.uk, not mine. That's on a single cam version of the engine, but it's the same I think.

 

Then mine, starting with ones of a new, whole tensioner, followed by some of the one I took a hacksaw to.

 

 

Comments and thoughts welcome.

 

Edited 19 December, 20196 yr by Wino

Just to clarify, the tiny spring mentioned in relation to the ball-bearing one-way valve is not the one shown anywhere in these pics. It's about 2.5mm diameter and about 3mm long, between the back of the ball bearing and the bit seen at the bottom of the piston bore.

That feeble-looking spring, if gummed up with oil sludge, could potentially prevent the one way valve seating properly and closing fully, I should imagine.

Edited 19 December, 20196 yr by Wino

An observation about the pmmonline pic: It looks from that shot as though the tensioner would hit a limitation after maybe 15-20mm of further piston travel as the chain elongates in service.  At that point the black hinged-at-the-top tensioner rail would seem to back up into the brown fixed rail and be incapable of further rightwards movement?

 

I concur with your observation, but...

If the chain is that stretched, surely its goosed anyway....

9 hours ago, mac11irl said:

I concur with your observation, but...

If the chain is that stretched, surely its goosed anyway....

 

A chain will continue to stretch until it eventually breaks, the design flaw here is the use of small sprockets which wear exacerbating chain stretch, because the chain run is so cramped the tensioner cannot take up much slack before reaching its limit of travel as Wino correctly points out.

I was aiming to bring in a bag of bits and pieces this morning to try to do some trigonometry and work out how much elongation it would take to allow that much movement at that point, but I forgot it.

I think the guide rails are ballpark 300mm long.  I just attempted a very crude calculation that came out as only about 1mm elongation so I don't think that can be right  .

Anyone else fancy a go?

 

 

I wonder @Wino - Are you applying chain stretch, but not wear (and possibly hooking) in the sprockets, and/or wear in the tensioner?

Edited 20 December, 20196 yr by KenONeill

I was just doing ridiculously oversimplified trigonometry.

 

There's an aggressively defended position in 'bike chain internet world' that says chains simply don't stretch at all, it's all about the wear at the pivot points making each link-to-link connection a little looser and longer. My gut tends to agree with that, hence why I refer to elongation, not stretch.  

18 minutes ago, Wino said:

I was just doing ridiculously oversimplified trigonometry.

 

There's an aggressively defended position in 'bike chain internet world' that says chains simply don't stretch at all, it's all about the wear at the pivot points making each link-to-link connection a little looser and longer. My gut tends to agree with that, hence why I refer to elongation, not stretch.  

 

Chains certainly do stretch, if you fit a brand new 1/8" chain to a single speed bike and press down hard on the pedals just once you'll need to adjust the chain tension again immediately, after a ride it settles down and stays in tension until wear starts to cause elongation.

So do you have to retension it every time you push down hard on the pedals…? 🤔

49 minutes ago, Wino said:

So do you have to retension it every time you push down hard on the pedals…? 🤔

 

Of course not, I assume you've seen the classic graph for Youngs modulus, like Bowden cable or guitar strings the initial stretching is soon taken up.

@Wino - I was considering the multiple sources of wear in a tensioned chain drive. I'll agree that the delta length in the chain itself is wear in the pivot holes and/or pins rather than exceeding the elastic limits in the links but the effect is still that a chain of length N becomes N + alpha.

1 hour ago, sepulchrave said:

 

Of course not, I assume you've seen the classic graph for Youngs modulus, like Bowden cable or guitar strings the initial stretching is soon taken up.

Hmm...

I'm not a qualified engineer, but I don't think I'd design a timing chain to be stressed beyond its elastic limit, ever. Seems unnecessary?

 

 

 

1 hour ago, Wino said:

Hmm...

I'm not a qualified engineer, but I don't think I'd design a timing chain to be stressed beyond its elastic limit, ever. Seems unnecessary?

 

I'm simply speaking from experience, I'm not suggesting that this system was designed that way and I agree that wear is the dominant factor in elongation I was trying to point out that there is very little room for adjustment and once the chain drive has bedded in during the first few hundred miles of use there is even less travel available to the tensioner to compensate for wear, leading to a loose chain and consequent timing slippage which is so common on these engines.

You don't need to be a qualified engineer to understand that it is common for chains and cables to be subjected to momentary loads which exceed the elastic limit and cause plastic deformation before the load drops back below the threshold, this is not dangerous and does not affect the ultimate tensile strength, hence my example about bicycle chains.

There is no evidence to suggest these cam chains actually snap, rather that the tensioner runs out of travel as you correctly observed. If larger drive sprockets had been used then the parallel chain run would be further apart allowing for more tensioner compensation, timing slippage would also be eliminated due to the chain being physically engaged with more teeth on the sprockets.

In regards to chain stretch as it were i do alot of mountain biking and like said the chain becomes worn at the links, ive replaced my chain after 500 miles and it has grown overall by half a link. 

Back to engine chains it does appear any vehicle with a chain suffers with chain issues.

Like i mentioned before i work on fords and they have started running wet timing belts again(run in oil) 

but a chain is a life time item which doesnt require being replaced like a belt.

Why dont they but intervals on them like belts which might help with so many premature engine failures. 

They obviously still cant get it right now with chains and with these longer service intervals dont help either

8 minutes ago, Damo152003 said:

a chain is a life time item which doesnt require being replaced like a belt

 

So an engine life time is ~30_000 miles then? (based on life before an engine with a cam chain and no tensioner becomes noisy).

1 hour ago, Damo152003 said:

In regards to chain stretch as it were i do alot of mountain biking and like said the chain becomes worn at the links, ive replaced my chain after 500 miles and it has grown overall by half a link.

 

Yes, but a mountain bike chain is not sealed in a case and fed clean oil whenever it's moving, consequently it wears very quickly, a modern derailleur chain is also made of high tensile steel where the elastic limit is extremely close to the ultimate tensile strength.