Belt drives have a major role in
rotary power transmission. Just try to peep under the hood of your automobile
whether it’s a car or a SUV, you are sure to find atleast one timing belt in
it. Here’s a typical application of the timing belt in an automobile.
This shows the usage of the belt from
high torque transmission to very low torque transmission applications.
Let us now look into the aspects that
make this timing belt so unique for rotary power transmission.
1. The timing belts provide a positive drive; by this we mean
that there is no slip and relative motion between the belt and the pulleys thus
maintaining a constant speed ratio or even a specific timed position.
2. The constructional aspect of the timing belt i.e. the
teeth in the belt help in transmitting large torques and are capable of
withstanding high accelerations.
3. The lesser cross sectional thickness of the belt enables
the belt to be more flexible and at times they are able to drive various
pulleys in the system very efficiently.
Constructional Aspects of the
Timing Belt
The timing belt is basically a flat
belt with evenly spaced teeth in its inner circumference. The teeth provide the
advantage that a gear or chain drive posses – positive transmission of rotation
and power.
As shown in the figure above, the
timing belt consists of
1. Steel Wire or the tension member – This is usually a wire
that takes up the load and provided the reinforcement to the belt. The various
materials that are used for the tension member are steel wire, glass fibres,
Kevlar and polyester cords
2. Base Material (Neoprene) – A good coat of neoprene is
provided to the tension member in order to prevent any damage to it by oil and
moisture. It also acts as a wear resistor. The teeth formed by the neoprene are
shear resistant as they are molded integrally along with the other elements of
the belt.
3. Nylon Coating – A film of nylon fabric coating is provided
on the external layers of the belt which provides a low co efficient of
friction for the surfaces that wear out quickly.
There are two types of tooth
shapes in the timing belt. They are
1. Trapezoidal Type – The trapezoidal type tends to have a trapezoidal
profile of the teeth that in turn mesh with a pulley possessing involute teeth.
The trapezoidal shape is actually that of an involute produced with an outside
diameter which is lesser than the pitch diameter.
2. Curvilinear Type – The curvilinear tooth profile is a circular arc. This
profile has some specific advantages like non tooth jumping due to increased
depth of the tooth in the pulley, higher contact area of the tooth and hence a
better power transmitting capacity, etc.
A comparison figure is shown below
indicating the profile types and geometry of the belt and the pulley
The belts are normally specified
based on the pitch of the teeth like 5mm, 8mm , etc. The width of the belt may
tend to vary based on the power or torque required to be transmitted.