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Timing Belts

Posted: August 12th, 2013, 11:25 am
by Will01
Timing Belts
Basic explanation

Timing belts are synchronous toothed belts. The teeth are to help synchronise power which makes them very suitable for cambelt applications. However they are also used on many others types of drive. There are other many different types e.g.
PU Belts – Industrial applications, used on some Automotive race applications e.g. supercharger drives
Rubber Belts – All applications including Automotive cambelts, auxiliary drives
There are many different companies who make them including Gates, Dayco & Conti etc.
All Rubber belts are not the same, many different manufacturers have many different constructions which suit different applications and environments e.g. low temperature, high mileage requirements, high torque etc. etc.
Belt Physicals
Primarily a belt is made up of 3 different component parts; Rubber, Cord (tensile member) & Fabric for teeth & sometimes a fabric for the back surface.
Some rubber belts do sometimes have a fabric on the back surface e.g. many VAG vehicles.
The 3 main individual components can have many different types and constructions themselves e.g.
Fabric can be thick, thin, have different types of weave, they may also have coatings applied as well e.g. PTFE (Teflon).
Rubber can be hard or soft and sometimes fibre loaded.
Cord can have glass fibre or Kevlar or carbon fibre etc. materials some can be high strength fibres, diameters and cord filaments can also be wound differently.
These individual components also have different adhesives to hold them together.
There are also different ways to produce belts.
Belt dimensions
Now to dimensions, a belt can have different number of teeth, pitches, width, length, tooth depth, tooth type, thickness & PLD
If we look at Tooth type there are trapezoidal forms & curvilinear forms of which there are multiple versions of each e.g. Trap (square tooth) Curvilinear HTD, HTD2, GT, Super Torque etc. etc. These also have their own tooth depths which are adjusted for different reasons e.g. tooth jump resistance, low noise as well as to fit carry over existing parts which is very common in the automotive industry.
This is where I specialise, although I can’t pretend to know everything about profiles as many of these were developed even before I was born! A major part of my day to day job is to recommend, develop, test & give technical support on Industrial/Automotive profiles.
Each of these tooth forms can also be tweaked to help improve meshing behaviour this in turn can be adjusted to help durability and NVH etc. This is a sort of art which is based on experience and testing.
Different pitches are used dependant on torque required to be transmitted e.g. 2 mm pitches (power steering systems) up to 3/8” for automotive applications (cambelts in particular) however in Industrial applications you can have smaller as well as much larger pitches e.g. 1mm up to 30mm.
Different thicknesses are also used dependant on constructions and requirements, in industrial applications sometimes double sided teeth are required e.g. twin power belts or sometimes mixtures of auxiliary and synchronous belts are made.
For belt length this is controlled with number of teeth and pitch, however you can also fine tune this in development and manufacture by varying diameters of moulds and production variability’s, this ultimately fine tunes the belt pitch and therefore length.
When we think about length in a synchronous timing belt in the automotive industry we also need to understand that belts do not stretch. This is a common misconception because people see the belt as a flexible piece of rubber. However what we know is that these belts have glass fibre, Kevlar, carbon fibre or steel cords which have a very high tensile strength.
In fact if we talk about length increase in a belt or even chain this is mainly down to wear e.g. chain links wear, in belts we can see wear on the fabric which ultimately changes its length in wrap.
This wear causes a change in PLD of the belt which changes the belt length in wrap, which changes the belts meshing performance, much like the wear of links in the chain which cause an increase in length of a chain.
In manufacture and development of belts and I assume in chain also this increase of length has to be factored in.
There are very strict criteria put down by engine manufacturers which both belt and chain have to abide by over their lifetime on a vehicle. Obviously you don’t want a belt or chain to elongate to such an extreme that it causes major changes in cam/crank timing. In belts we are talking about changes of length of less than 0.1%.

PLD is a difficult one to explain, to be honest I am not going to try and do this as you need to have more in depth understanding in this subject. But to give you an idea Pulleys, Mould tooling and Belt each have their own PLD. On any one drive these could all be different to achieve different factors e.g. durability, NVH performance.
PLD diametrically in mould tooling and pulleys is the difference between the pitch circle diameter (this can’t be measured in a pulley or mould as it is outside the OD and therefore is in space, so can only be calculated) and the outside diameter (this is the outer most part where the tooth profile tangents the outside diameter).
PLD normally denoted radially in belts is difficult to explain but an easy way of thinking about this is to think that the PLD radially of a belt is the centre axis of the belt when the belt is in wrap. This can be measured in different ways and is a very complicated subject, made harder because a belt is a composite flexible component.
This is just to give everyone a little insight into belts and help to understand that one belt on one engine is not necessarily the same as a belt on another engine, even if you think the belt looks the same.

Re: Timing Belts

Posted: August 12th, 2013, 11:25 am
by Will01
Handling belts a few basic comments
I thought I would also share a little info on handling of belts.
We see it so often that people pickup belts and then compare the length of a belt by pulling between their fingers. The main problem with this is, it can destroy the belt as if pulled hard enough it can cause excessive bending of the tensile member causing small breaks. This can then lead to drastic failure of the belt by tensiling.
We also see belts so often that have water or oil soaked into them. Again for most belts this can destroy them because it can break down adhesions in the belt leading to tooth shear, also this can break the tensile members through hydraulic action.
However although I say this we are now starting to see more and more belts being run inside engines which is normally where you would see a chain. These belts are specifically manufactured and developed to work in this environment and what has been explained above about oil etc., this can be mostly ignored.
Another thing we see all too often is mechanics using tools to lever belts onto drives because it is tight, mainly because proper installation has not been followed. Unfortunately again this can cause damage to the belt which can lead to failure.
Belts are designed to work at set tensions and again another thing we see all too often is the failure to install belts at their correct tension for the drive. This can lead to a multitude of failures, which is visible to us when we get belts back.
After all we design and make the belts and they are tested through thousands of hours of testing, tests are designed to fail the belt in every possible way so we see everyway how the belts fail. This helps us to validate belts for the market on set engine platforms.
People still see belts as flexible things that break, this is normally due to installation errors or use. It is important to remember belts are designed to run on standard engines within normal production tolerances. If you use a high lift cam or longer stroke crank or anything that changes the loading on the belt compared to standard then you must accept that the belt is not going to last as long as the standard unit. Allowances must be made, and therefore the normal change period will not apply.
The idea that chains are superior is something that is hard to argue as in many cases belts have had to be used where chains have not been suited due to failures.
For many years belt companies have fought against chains. I am sure most of you have seen there are more belts than ever on an increasing range of vehicles.
We are seeing vehicle manufacturers more and more now asking for belts to last the lifetime of the vehicle e.g. 240,000km’s, 300,000km’s in much more harsher environments e.g. low temperatures as low as -40°C. Belts do last longer than they used to and they will continue to improve with time. Many belts now last longer than other more major components on an engine. In many cases out living the metal parts they work with.

Re: Timing Belts

Posted: August 12th, 2013, 11:25 am
by Will01
Installation/Service of belts
This is quite a big subject and to be honest I don’t get too caught up with this side of things, although I have done a fair few cambelt changes in the past.
Some of what I have explained above covers this, but to explain when fitting a cambelt you should always read the instructions first. Books like Autodata give you all the information you need, make sure you always use an up to date copy. Sometimes we are asked to correct belt installation procedures if we find that a particular problem is apparent in the aftermarket sector. This may not be anything to do with belt failure, but most likely due to installation errors causing complaints. This makes it more important to check you have an up to date copy.
If you have ever changed a timing chain you wouldn’t twist it and bend it or manipulate it with tools to install it, so again the same can be said for belts.
If you have ever removed a belt I would recommend that you do not re-fit the belt under any circumstances. I would always recommend replacing with a new belt. At the end of the day it is a relatively cheap component, but failure can cause catastrophic damage of an engine. I would also recommend that a belt is replaced as part of a kit e.g. with new tensioners and idlers where applicable. It’s pointless fitting a new belt on old idlers or tensioners. The last thing you want to happen is an idler or tensioner fails and you have to replace the belts again with more expense.
If you are going to do a job, its best to do it right first time.
More common now than previously, other items are being driven by the cambelt e.g. waterpump, injection pumps etc. etc. Again if this is the case I would recommend you replace serviceable components at the same time to help avoid doing at a later date and having to replace the belt again. Waterpumps do fail, most commonly due to bearing damage. If hate to admit it but in the past I have changed a timing belt and left the waterpump thinking it would be alright and then the bearing seized which caused the breakage of the belt. It then took a very long time and much expense to rebuild the engine.
Always make sure the belt is correctly tensioned. If there is an installation procedure, follow it. If you need specific tools e.g. tension meter, use them. Belt failure can result from being incorrectly tensioned. I have seen belts that have been so over tightened they were like piano strings. I have also seen belts so under tensioned they could be pulled off the drive without any tools.
Part of the drive design tools we use for different drives actually give us technical details on installation tension to suit different duty cycles. On certain types of drive that I have been involved in we give frequency measurements for tensioning belts, if this is not suitable for various reasons we also can calculate tension for deflection with certain force applied. In one off drives this is a preferred method as it uses commonly available tools. But Sonic tension meters are also available which measure the frequency of a span which is struck. Much like a tuning gauge for a guitar.
More modern vehicles can have very complicated timing belt installation procedures. Sometimes even sonic tension meters have to be used to check the correct belt tension has been setup.
Always make sure you follow the mileage or time service intervals, if you belt is overdue a change by time or mileage you are on borrowed time. They are designed to last a certain amount of time. There are many environmental factors that can affect this also. We have seen belts that have been sat on engines for years without moving that have been in worse condition than a belt that has run for the same time period.
There are specific ways to store belts and they do have a shelf life. I know in the classic car market some people like to buy new old stock. But would you really want to buy an original timing belt for your Fiat that was made in the 70’s? I for one wouldn’t!
I also wouldn’t buy cheap belts, unfortunately more and more we are starting to see counterfeit belts on the market that are sold on the internet on well-known sites. Don’t get me wrong the supplier doesn’t always know they are counterfeit, even we find it difficult at times when looking at a complete belt. On closer physical testing it is easy to see, but who is going to cut a belt after buying it and before fitting to a vehicle, you can’t stick them back together again. What I am trying to say is be careful where you buy your parts.

I hope it can go someway to help people understanding timing belts. It is not meant as a training manual, just merely a bit of guidance. Over the course of time I may add a little more detail as and where applicable.

Re: Timing Belts

Posted: August 12th, 2013, 11:27 am
by Will01
Just a quick comparison for people to get a basic idea of differences between Belt and Chain pulleys & sprockets

Re: Timing Belts

Posted: August 12th, 2013, 11:31 am
by Will01
I have also added a quick spreadsheet to allow people to be able to do quick dimension checks on pulleys. Also to give people a rough idea how to calculate dimensions of a pulley.
This is for information only and i take no responsibility for people making mistakes using this.
It is meant purely to aid people and to help their understanding.
Please bear in mind that for many different reasons application pulleys on engines may not be standard, as they may have been tuned for improved performance etc.

Re: Timing Belts

Posted: April 13th, 2016, 9:42 pm
by Will01
This may help for Biancorally. Just a wee bump to make it easier to find.

Re: Timing Belts

Posted: April 14th, 2016, 5:05 pm
by Guy Croft
Just been re-reading your superb and expert thread Will,

thank you very much indeed for writing this,


Re: Timing Belts

Posted: April 14th, 2016, 6:07 pm
by Will01
No problem Guy, where possible i like to try and help.
I only wish i could share more.
If anyone has any questions just let me know

Re: Timing Belts

Posted: April 14th, 2016, 7:50 pm
by Guy Croft
dear Will,

this post has been viewed over 2000 times.

Running a forum has taught me one rarely gets any thanks or recognition. Readers, Will works for the world's leading belt-drive manufacturer Gates, in Dumfries.

But anyway, I own the forum and I appreciate the post, for sure.

Yes it is a pity more folk don't check in and at least say thanks, guess that is how it is these days, knowledge is free. Real knowledge like yours. Haha!!!

I shut down access to photos, to non-paying members two years ago. There are times, yes, I feel like shutting the forum down altogether!

If you liked this post, readers, at least have the common decency to say so here. I did not ask Will to write it and he certainly did not get paid for it, despite that this kind of thing has NEVER been published before.

Those who thing (believe) this forum is not worth £35 for life of site - I am glad you cannot see the photos tbh...

Thank you.

Very sincerely,


Re: Timing Belts

Posted: April 14th, 2016, 8:11 pm
by Biancorally
Hello Will,
I am very glad that you gave so detailed and interesting information about belts.
You are an expert and it is great that you share your knowledge on the forum.
The belt world is so complex... your post will be more than useful (not only for me).
I asked you questions, and you are really helping me a lot! I really appreciate.

Many Thanks, Will!!

Re: Timing Belts

Posted: April 14th, 2016, 8:36 pm
by Will01
You are both too kind
I am certainly no expert in belts, there is still much to learn.
As long as there is plenty to learn it will continue to be interesting.

I think the important thing about synchronous belts is we shouldn't just look at the belt, we should look at the complete drive. Idlers, tensioners, pulleys, placement of components, installation tension and everything in between.
In the OE world we spend years finessing these drives for the customer, we work with teams across the world and have many people with their own expertise and pull together. It simply is not possible to have one person do everything now these days.

There are basic design considerations that we can adopt to make things easier, this is what i have started to touch on with this thread.
The recent posts have actually reminded me that i should continue with adding more detail on the rest of the system.

Take it easy and thankyou for your kind comments :)

Re: Timing Belts

Posted: April 14th, 2016, 8:50 pm
by Will01
Next subject to follow Sprocket/Pulley design.
Again i cannot share too much technical detail, but i would like to share some info which is in the public domain and in the automotive industry and is accepted as standard.
I know there are many of you out there who do machine your own sprockets, so i hope it will be of help.

Re: Timing Belts

Posted: April 14th, 2016, 8:55 pm
by Biancorally
Thanks Will,
I am sure it will be very interesting for all of us!

Re: Timing Belts

Posted: April 14th, 2016, 9:12 pm
by Rallyroller
Hi Will

Great piece.

As an engineer i have worked with belts at times during my career in industrial applications. However normally it was choosing it from a catalogue with limited information. As usual with most things in life, what appears to be a simple topic is far from it. Your insight makes an interesting and educational read.

I spoke to Guy some time ago regarding my industry- and he suggested writing a piece for the forum. I think you may have embarrassed me into doing something.

Re: Timing Belts

Posted: April 14th, 2016, 9:56 pm
by Guy Croft
(Will. No expert on belts...hahah!!!)

He is actually very expert.