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Important Note:..The notes below are intended to be concise informative guidance notes. Manufacturers
literature and the relevant standards provide the necessary detailed information required for detail design.
I have included links to sites providing good quality information on this topic.
Vee belt drives replaced flat belt drives for many applications because higher power
drives could be transmitted with more compact drive arrangements. With a
flat belt drive only one belt is used. With a vee belt drive a number
of belts are used. Flat belts and vee belts may, and do,slip as the loading
increases. For belt drives which drive without slip timing belts should
be used. Vee belts on higher power duties generally
have to be matched to ensure the drive power is shared.
Vee belts are generally manufactured from a core of high tensile cord in a synthetic rubber matrix enclosed
in a fabric reinforce rubber lining.
The vee belt sections and the pulley groove dimensions are in accordance with British /European standards.
**The mass / m vary from different reference sources.
Vee belt power capacity
The power transmission rating of a vee belt drive is based on the belts used, the number
of belts, the angle of wrap of the belt on the smallest pulley,and the rotational speed of the pulleys.
The rating required is selected as the product of the design load and an appropriate
N = Pd. K / Pb.F.E
Angle of contact factor
The angle of contact ( θ) of the vee belt over the smallest pulley when
the pitch dia of the largest and smallest pulleys are (Dp & dp)
and the centre distance = C is provided below:
The angle of contact factor F varies from 1 for an angle of contact of θ of 180o down to
0,82 for and angle of contact θ of 120o.
The pitch length of belt may be calculated (approximately)from the equation below
L = 2.C + p (D p + dp)/2 + (D p - dp)2 /4.C
or more accurately using
L = 2.C.cos A + p (D p + dp)/2 + (D p - dp).A
where A is in radians and sin A =(D p - dp)/2.C
The power ratings below are based on the smaller pulley with 180o of belt wrap !!.
The ratings are based on a light duty application for a transmission working for less than
10 hours per day i.e a service factor K= 1. At the other extreme of a extra heavy duty drive e.g.direct on
line AC motor with frequent stops and starts, working for 24 hours /day a service factor K = 1,8
would be necessary. The basic power ratings shown below are used to establish the
drive power capacity of a belt drive system using a formula.
When designing belt drives it is normal to apply a service factor to the drive operating
load to compensate for allow for different driver type, driven load types and operating
periods. Typical service factor values are included on the linked page Service Factors
If two pulleys of different diameters are used then in driving to the limit
the belt will slip on the smaller pulley first
The power transferred =
P = (T1 - T2 ).v / 1000
Links to Belt Design
Send Comments to Roy Beardmore
Last Updated 22/01/2013