How to adjust timing

[T-Cut]

I'm no expert in cam belt, or any other belt technology, but I do know the physics of stretching and elasticity, so I'll offer my view on it.

Originally Posted by mh007

Although belts are lined with reinforced strands (when these break, the belt snaps), they are bound to expand/contract to a degree surely & as they're getting to the end of their life, the degree of contraction gets less & less.

Almost all materials stretch if enough force is applied to them. If this increase in length occurs within the elastic limit of the material, then it will regain (shrink) when the force is removed. In the case of belts, the force is along the length of the belt between the driving pulley/sprocket and the driven one. The Wikipedia reference to how belt stretch retards the valves describes a transient effect which disappears when the revs (force) is reduced. The timing at low revs/force isn't different from before. All the stretching is taking place within the designed elastic limit. The same principle applies to many other situations like head bolts and handbrake cables. If the handbrake cable didn't stretch when applied, it wouldn't work. After it's released, the cable is the same length as it was. However, if enough force is applied to exceed the elastic limit, there is no hysteresis when the force is removed. In other words, the stretched size is permanent. A cable or bolt in this condition won't work properly and simply gets longer as force is applied.

With regard to 'slack' belts after a few years sevice, the question is whether they have permanently stretched or has something 'worn' smaller to make them slack? I suspect there's a bit of both, but I doubt the stretch factor is especially significant. Since a belt goes around a fixed circuit, the amount of wear per tooth could be a more important element in slackness than is a true increase in the cord lengths. To illustrate this, I'll ask a simple question I remember from school.

A man has a big ball of unstretchable string which he fastens to a stick hammered into the ground. The stick is on the Earth's equator. He walks around the equator unrolling the string so it touches the ground all around the Earth. When he gets back to his stick, he pulls the string tight and ties this end to the stick as well. That's 24,000 miles of tight string.

Now here's the relevant part. The man cuts the tightened string with his penknife and inserts 1 yard of extra string into the gap. Naturally, this makes the string around the equator slack, because it's now 24,000 miles plus 1 yard. The man then asks millions of his mates to go around the equator and lift the string up from the ground and hold it above the Earth's surface equally all round.

How high will it be off the ground? (Guess if you don't know.)

I think exactly the same principle applies to belts. The question is, how much of that 'yard' is stretch and how much is inter-tooth wear?

TC

[mh007]

Originally Posted by chrissyboy

hi , like all with all things they are designed to do certain things but in reality do do uexpected things .your opinion is based of what you have seen in your years in the trade.just like some posts i have done re the hgf on the 2.5 engine .. yes they do go andnot as rare as members state ... i myself have never done any internal work on an engine .i used to fix up cortinas whe younger ,no problem changing an engine or gear box or clutch ,cars changed and i never kept up as i started doing roofing and building work..i have now gone back to cars at the 53 years old .im training ondiagnostics but also now know how to change disc`s and pads ,as your know the cortina was really easy conpared to the model cars... can also service cars from panda1s to jags, now the jag is a pig to service as removal of the inlet manifold is required to change left back plugs. as i said seeing i believeing and in your case i will bow to your findings given the extensive exprience you have cant ove the years ....

i do know that the belts now are cover with some new stuff that reinforces them making them less likely to snap or indeed stretch . but there is always the chance of the belt becoming strecthed due to what ever reason .

Thanks Chrissyboy for your comments. Good to hear you couldn't stay away from the spanners !!

Ford Cortina's....that takes me back, I started off on the old side-valve & overhead-valve engines, Ford Prefects, Anglia's & Morris 1000's (& some earlier stuff) !!

Over the years i've worked just about anything & everything (with exceptions of course)

I'm now more involved in Diagnostics but it was so much easier in the days of old !!!

Sorry about going off-thread everyone.....just re-living old memories.

[3disco]

Originally Posted by T-Cut

I'm no expert in cam belt, or any other belt technology, but I do know the physics of stretching and elasticity, so I'll offer my view on it.



Almost all materials stretch if enough force is applied to them. If this increase in length occurs within the elastic limit of the material, then it will regain (shrink) when the force is removed. In the case of belts, the force is along the length of the belt between the driving pulley/sprocket and the driven one. The Wikipedia reference to how belt stretch retards the valves describes a transient effect which disappears when the revs (force) is reduced. The timing at low revs/force isn't different from before. All the stretching is taking place within the designed elastic limit. The same principle applies to many other situations like head bolts and handbrake cables. If the handbrake cable didn't stretch when applied, it wouldn't work. After it's released, the cable is the same length as it was. However, if enough force is applied to exceed the elastic limit, there is no hysteresis when the force is removed. In other words, the stretched size is permanent. A cable or bolt in this condition won't work properly and simply gets longer as force is applied.

With regard to 'slack' belts after a few years sevice, the question is whether they have permanently stretched or has something 'worn' smaller to make them slack? I suspect there's a bit of both, but I doubt the stretch factor is especially significant. Since a belt goes around a fixed circuit, the amount of wear per tooth could be a more important element in slackness than is a true increase in the cord lengths. To illustrate this, I'll ask a simple question I remember from school.

A man has a big ball of unstretchable string which he fastens to a stick hammered into the ground. The stick is on the Earth's equator. He walks around the equator unrolling the string so it touches the ground all around the Earth. When he gets back to his stick, he pulls the string tight and ties this end to the stick as well. That's 24,000 miles of tight string.

Now here's the relevant part. The man cuts the tightened string with his penknife and inserts 1 yard of extra string into the gap. Naturally, this makes the string around the equator slack, because it's now 24,000 miles plus 1 yard. The man then asks millions of his mates to go around the equator and lift the string up from the ground and hold it above the Earth's surface equally all round.

How high will it be off the ground? (Guess if you don't know.)

I think exactly the same principle applies to belts. The question is, how much of that 'yard' is stretch and how much is inter-cog wear?

TC

Would it not depend on his mates being all the same height?

[mh007]

Originally Posted by 3disco

Would it not depend on his mates being all the same height?

Now that was good Timing

[T-Cut]

Originally Posted by 3disco

Would it not depend on his mates being all the same height?

Err, they're all exactly the same height.

Nobody wanna guess then. It explains slack cam belts.

TC

[pab]

Originally Posted by mh007

Didn't think of that Zeb !

Situated by the Battery on the 1.8 if I remember rightly.

The electronic tappet simulator springs to mind..

Have a search purge valve.

[chrissyboy]

Originally Posted by mh007

Thanks Chrissyboy for your comments. Good to hear you couldn't stay away from the spanners !!

Ford Cortina's....that takes me back, I started off on the old side-valve & overhead-valve engines, Ford Prefects, Anglia's & Morris 1000's (& some earlier stuff) !!

Over the years i've worked just about anything & everything (with exceptions of course)

I'm now more involved in Diagnostics but it was so much easier in the days of old !!!

Sorry about going off-thread everyone.....just re-living old memories.

your not kidding about it being easier inthe old days .... had a primera in that was coming up with cam sensor fault ,when starting in the morning it was revng like the maf was no good .... in the end decided to check the fuel pump by removing the fuel pump fuse and replaceing with a fuse with a wire loop on it so i could clamp a current clamp on it to see the was is was drawing ,,foe 2 days the car didnt do it ,then the 3rd day the voltage was down to 3 from 7 so the fuel pressur was the problem ,just goes to show car says one thing yet the problem was miles away still new pump fitted and problem solved...

[COLVERT]

Originally Posted by T-Cut

Err, they're all exactly the same height.

Nobody wanna guess then. It explains slack cam belts.

TC

Pi. times D. ??????
Circumference being 24,000miles
and circumference being 24,000 miles and a yard.

Oh and Pi being-----3.141592653589793238462643383279502884 approx. Lol.






Colvert.

[T-Cut]

Well since nobody guessed right, the answer is about 1 foot (mathematically it's 1 yard divided by pi). So increasing the 24,000 mile belt by a yard causes it to slacken on the pulley by a foot all round. It doesn't matter what the original length is, the slack is always 1yard/pi. If you translate that to a cam belt and each tooth wears by, say 10 thou, then the belt will get pretty slack compared with its original fitting. The belt hasn't stetched. it's slack because a number of minor wear areas contribute to a significant change in the running circumference/backlash.

TC

[marinabrian]

Originally Posted by mh007

I completely disagree Brian, any belt, Sychronous or otherwise, can stretch !
I can provide technical info on this if you would like to see it but I've seen plenty throughout my years in the trade.

Is this the sort of technical info you mean? http://www.gates.com/facts/documents/Gf000287.pdf
All Rover group products use Gates HTD belts as O.E. and "stretching" is not one of the criteria, wear to the running surfaces of the belt/pulleys are the factors in play here coupled with incorrect tensioning procedures during installation.
Now I may be only an amateur mechanic, however I am a professional engineer, & certainly know a thing or two when it comes to power transmission.
Interesting reading.
Brian