Timing Belts
Posted: August 12th, 2013, 11:25 am
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.
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.