spospe Posted March 20, 2013 Share Posted March 20, 2013 Timing belts versus chains. There have been a few threads recently regarding the problems of timing belts and when they should be renewed. The following are my thoughts and personal experiences of all three common methods of driving camshafts in internal combustion engines. Firstly and most rarely used is gear drive. This is an excellent method which does not rely on either oil pressure for tension, nor does it require any replacement of parts throughout the service life of the unit. The example that comes to mind is the MG PA and PB of the mid 1930’s. Both engines were overhead camshaft and were driven via the vertically mounted dynamo using bevel gears at both ends of the armature. Secondly is the chain driven type, often trumpeted as being superior to the fabric belt type of drive. The only breakage that I have ever suffered in 50 years of motoring was when my wife’s 1.3 petrol Yaris snapped its chain whilst descending a hill at about 30 mph in 3rd gear and wrecked the engine. Chains can suffer from a lack of regular oil changes and (especially) wear of the (usually) oil pressure regulated chain tensioner. The most common type of failure of the chain system is when a vehicle so fitted is ‘bump started’. Under these conditions, the oil pressure is very low / non existent and the chain is slack. The sudden shock load of engaging the clutch in a too low gear causes the chain to jump over the teeth of one or more of the chain wheels, leading to a drastic alteration of the timing and possibly the pistons hitting the valves. If you have to bump start a chain driven engine then a, use 4th gear (with a 5 speed gearbox, or even 5th ) and feed the clutch in slowly so as to let the engine run up smoothly and generate some oil pressure and chain tension before it tries to fire up. The most common engine to suffer from a failure to bump start smoothly is the current Ford Transit 2.4 diesel. Thirdly fabric belts are commonly used as a cheaper alternative to the two above types. Belts do not suffer from lubrication problems and normally are tensioned by springs, so avoiding the problem faced by the chain type when bump starting. The service life of a belt does vary and I have seen some (Ford) types that have a quoted 10 year / 100,000 mile life. All the above types can be used to drive both the camshafts and the high pressure injection pump of diesel engines. Best advice for all types is simple: carry out manufacturers recommended servicing at the recommended intervals. Link to comment Share on other sites More sharing options...
Paul- Posted March 20, 2013 Share Posted March 20, 2013 When a chain needs replacing it will let you know by rattling, don't ignore it. the garage will tell also tell you when its ready to be replaced when they check it on the service. When a belt needs replacing it will let you know by making a loud noise like a propeller as it shreds then within seconds you'll hear the noise of the engine as it self destructs. This usually happens in the 12 months before renewal time :-( Link to comment Share on other sites More sharing options...
spospe Posted March 20, 2013 Author Share Posted March 20, 2013 Paul- - 2013-03-20 7:26 PM When a chain needs replacing it will let you know by rattling, don't ignore it. the garage will tell also tell you when its ready to be replaced when they check it on the service. When a belt needs replacing it will let you know by making a loud noise like a propeller as it shreds then within seconds you'll hear the noise of the engine as it self destructs. This usually happens in the 12 months before renewal time :-( When the chain in the Yaris let go, there was no warning whatsoever. Link to comment Share on other sites More sharing options...
laimeduck Posted March 20, 2013 Share Posted March 20, 2013 spospe - 2013-03-20 9:13 PM Paul- - 2013-03-20 7:26 PM When a chain needs replacing it will let you know by rattling, don't ignore it. the garage will tell also tell you when its ready to be replaced when they check it on the service. When a belt needs replacing it will let you know by making a loud noise like a propeller as it shreds then within seconds you'll hear the noise of the engine as it self destructs. This usually happens in the 12 months before renewal time :-( When the chain in the Yaris let go, there was no warning whatsoever. Same on my Jaguar V8, one chain went, bye bye jaguar :'( Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 21, 2013 Share Posted March 21, 2013 All vehicle motors have moving parts, and any 'machine' with moving parts has the inherent potential to break. Timing belts and timing chains can, and do, fail catastrophically, but received wisdom is that the former fail more frequently than the latter. Given the choice, on a road vehicle, I'd opt for a timing chain over a timing belt, but it would be unrealistic to believe that a timing chain is bomb-proof. In principle, an overhead-camshaft drive-system using a metal gear-train should be most reliable, but there's a tendency for this arrangement to be noisy unless great care is taken to address the 'cogs' changing clearances when the motor warms up. Volkswagen's V10 diesel motor, with gear driven cams, allows for this, but the motor's design is complicated as a result. The Wikipedia entries for "timing belt" and "camshaft" summarise the various systems well: http://en.wikipedia.org/wiki/Timing_belt http://en.wikipedia.org/wiki/Camshaft In his original posting spospe mentions an overhead camshaft driven by a bevel-gear/shaft arrangement. This drive-system was quite common in the past for highish performance motorbikes - 'cammy' Nortons and Velocettes in the UK, and Ducati in Italy. The bottom photo on this webpage http://www.householdgods.co.uk/ducatidaytona/pages/engine.htm clearly shows the bevel-gears in the cylinder head. I owned a 1964 Ducati Daytona for a while (one of the reasons why I've a phobia of Italian engineering!) and the camshaft drive was very noisy. Didn't matter with a motorbike, but would have been intolerable with a car. Link to comment Share on other sites More sharing options...
Steve928 Posted March 21, 2013 Share Posted March 21, 2013 spospe - 2013-03-20 2:38 PM Firstly and most rarely used is gear drive. This is an excellent method which does not rely on either oil pressure for tension, nor does it require any replacement of parts throughout the service life of the unit. The example that comes to mind is the MG PA and PB of the mid 1930’s. Both engines were overhead camshaft and were driven via the vertically mounted dynamo using bevel gears at both ends of the armature. The engine in my boat (a marinised Peugeot 2 litre diesel) has maintenance-free gear driven camshaft and injection pump. The base Peugeot engine had chain-drive but Volvo Penta thought even that to be too unreliable and engineered the gear-drive. I guess that the move to overhead cams is what has left us with belt or chain drives, although the example you give shows it can be done.. Link to comment Share on other sites More sharing options...
spospe Posted March 21, 2013 Author Share Posted March 21, 2013 Derek Uzzell - 2013-03-21 7:45 AM Timing belts and timing chains can, and do, fail catastrophically, but received wisdom is that the former fail more frequently than the latter. Given the choice, on a road vehicle, I'd opt for a timing chain over a timing belt, but it would be unrealistic to believe that a timing chain is bomb-proof. This is my view exactly, a chain drive will normally be more reliable (given proper servicing and oil changes) than a fabric belt, even though the only failure I have experienced, has been to a chain. Talking to my brother-in-law who has his own engine remanufacturing business, the most common cause of belt failure is neglect (missed routine belt changes). On some engines, the Ford 2.5DI for example, best practice is to change the belt tensioner at the same time as the belt and failure to do this can result in failure, even though the belt itself has been replaced. Link to comment Share on other sites More sharing options...
Tin Man Posted March 21, 2013 Share Posted March 21, 2013 Jus had mine done. Just 5 years old with 18000 miles completed. Followed the advice given on this forum. Link to comment Share on other sites More sharing options...
Paul- Posted March 21, 2013 Share Posted March 21, 2013 spospe - 2013-03-20 9:13 PM Paul- - 2013-03-20 7:26 PM When a chain needs replacing it will let you know by rattling, don't ignore it. the garage will tell also tell you when its ready to be replaced when they check it on the service. When a belt needs replacing it will let you know by making a loud noise like a propeller as it shreds then within seconds you'll hear the noise of the engine as it self destructs. This usually happens in the 12 months before renewal time :-( When the chain in the Yaris let go, there was no warning whatsoever. Usually the belt doesn't make a noise, I was referring to my peugeot, sorry :$ Its the serpentine belt that shreds, the shredded parts wrap around the cam belt & pull it off, both of the times it happened to me I was hardly moving, once when reversing and the second when I was in a slow moving traffic coming off a ferry. Both times I got away with it as I turned the engine off before any damage was done ;-) Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 22, 2013 Share Posted March 22, 2013 Paul- - 2013-03-21 7:46 PM Its the serpentine belt that shreds, the shredded parts wrap around the cam belt & pull it off... As I understand it, what you seem to be suggesting is that timing-belts themselves don't normally fail; it's an ancilliary belt that fails and that affects the timing-belt. While that certainly can happen - apparently it has happened twice to you - it's much more usual (as explained in the "Failure" section of the Wikipedia "timing belt" entry I gave a link to earlier) for the timing-belt itself to 'shred' or for teeth on the belt to be stripped. Although one might well get away with it if an ancilliary belt starts to affect the timing-belt, if the latter seriously delaminates or loses teeth, with an interference engine design (as vehicle diesel motors have) it's pretty much inevitable that the valves will contact the pistons. How much harm will then be caused will be pure pot-luck but, if the motor is turning fast, the chances are high that the damage will be severe. Link to comment Share on other sites More sharing options...
Paul- Posted March 22, 2013 Share Posted March 22, 2013 Derek Uzzell - 2013-03-22 7:40 AM Paul- - 2013-03-21 7:46 PM Its the serpentine belt that shreds, the shredded parts wrap around the cam belt & pull it off... As I understand it, what you seem to be suggesting is that timing-belts themselves don't normally fail; it's an ancilliary belt that fails and that affects the timing-belt. While that certainly can happen - apparently it has happened twice to you - it's much more usual (as explained in the "Failure" section of the Wikipedia "timing belt" entry I gave a link to earlier) for the timing-belt itself to 'shred' or for teeth on the belt to be stripped. Although one might well get away with it if an ancilliary belt starts to affect the timing-belt, if the latter seriously delaminates or loses teeth, with an interference engine design (as vehicle diesel motors have) it's pretty much inevitable that the valves will contact the pistons. How much harm will then be caused will be pure pot-luck but, if the motor is turning fast, the chances are high that the damage will be severe. No Timing belts do fail they shred, but usually its the teeth that strip, not all vehicles are effected by the auxiliary belt failure puling the cam belt off either , however some Peugeots were and this is in addition to cam-belt failure. I was fortunate as both times the cam-belt only jumped a couple of teeth before I turned the engine off, if it had happened while moving at speed, it would indeed have done the damage you suggest. Cam-belts can be susceptible to damage from many sources and thats why I prefer timing chains ;-) Link to comment Share on other sites More sharing options...
Guest Peter James Posted March 22, 2013 Share Posted March 22, 2013 Paul- - 2013-03-22 10:27 AM usually its the teeth that strip, Since the teeth are not held on by the fabric reinforcement, only by the 'rubber', and its the 'rubber' that ages, that sounds logical to me :-) Maybe when the teeth start coming off, the debris strips the belt? Link to comment Share on other sites More sharing options...
Derek Uzzell Posted March 22, 2013 Share Posted March 22, 2013 Peter James - 2013-03-22 12:18 PM Paul- - 2013-03-22 10:27 AM usually its the teeth that strip, Since the teeth are not held on by the fabric reinforcement, only by the 'rubber', and its the 'rubber' that ages, that sounds logical to me :-) Maybe when the teeth start coming off, the debris strips the belt? These links may interest you or (if your vehicle has a timing-belt) give you sleepless nights: http://www.mlcmotorfactors.co.uk/Trouble%20Tracers/TimingbeltTT.pdf http://www.motoringassist.com/motoring-advice/technical-advice/your-guide-to-servicing-timing-belt-replacement/ (If you mean by "Maybe when the teeth start coming off, the debris strips the belt?" that teeth coming off the belt may result in the rest of the belt shredding, breaking or jumping off the pulleys, I shouldn't worry too much about that scenario. It would be like Mary Queen of Scots worrying whether she would lose her voice when the executioner cut off her head.) Link to comment Share on other sites More sharing options...
Muswell Posted March 22, 2013 Share Posted March 22, 2013 Oh yippee, thanks. Something else to add to my list of things to worry about. I remember riding my old 2 stroke motor-bike up motorways and passing the time by hearing noises and wondering if the engine was going to seize, which they did, and throw me into the path of the following traffic. I recall it was recommended to keep your fingers on the clutch lever just in case. Link to comment Share on other sites More sharing options...
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