Belt Drives

A belt is a looped strip of flexible material used to mechanically link two or more rotating shafts. A belt drive offers smooth transmission of power between shafts at a considerable distance. Belt drives are used as a source of motion to transfer to efficiently transmit power or to track relative movement.

Types of Belt Drives

Belts are classified in many types according to usage, position, shape like flat, v-belt, round ropes, etc., but Belt drives are different from the belts, these are described as the combination of pulleys according to their position and also their carrying or transmitting power from one pulley to another pulley.

Belts drives are classified as

1. Simple belt drives
2. Compound belt drives

1] Simple belt drives

In this flat belt, it can transmit the power from one pulley to another pulley where the axes of pullies are parallel to each other and again these flat belt drives further classified into open and cross belt drives.

• 1.1) Open belt drives

  An open belt drive is used to rotate the driven pulley in the same direction of driving pulley as shown in the figure below.

  

  In the motion of belt drive, power transmission results make one side of pulley more tightened compared to the other side. In horizontal drives, the tightened side is always kept in the lower side of two pulleys because the sag of the upper side slightly increases the angle of folding of the belt on the two pulleys. The figure below shows an open belt drive nomenclature.

  

  $\mathrm{d}_{\mathrm{L}}-$ Diameter of the larger pulley

  $\mathrm{d}_s$ - Diameter of the smaller pulley

  $\alpha_L$ - Angle of wrap of the larger pulley

  $\alpha_s$ - Angle of wrap of the smaller pulley

  $\mathrm{C}$ - Center distance between the two pulleys

  Basic Formulae

  $\alpha_L = 180^{\circ}+2 \beta$

  $\alpha_S = 180^{\circ}-2 \beta$

  Where angle $\beta$ is,

  $\beta=\sin ^{-1}\left(\frac{d_{L}-d_{s}}{2 C}\right)$

  $\mathrm{L}_o$ = Length of open belt

  $L_{o}=\frac{\pi}{2}\left(d_{L}+d_{s}\right)+2 C+\frac{1}{4 C}\left(d_{L}-d_{s}\right)^{2}$

  
  

• 1.2) Crossed belt drive

  A crossed belt drive is used to rotate driven pulley in the opposite direction of the driving pulley as shown in the figure below. Higher the value of wrap enables more power can be transmitted than an open belt drive. However, bending and wear of the belt are important concerns.

  

  The figure below shows a cross belt drive nomenclature.

  

  $\mathrm{d}_L$ - Diameter of the larger pulley

  $\mathrm{d}_S$ - Diameter of the smaller pulley

  $\alpha_L$ - Angle of wrap of the larger pulley

  $\alpha_S$ - Angle of wrap of the smaller pulley

  $\mathrm{C}$ - Center distance between the two pulleys

  Basic Formulae

  $\alpha_L = \alpha_S =180^{\circ}+2 \beta$

  Where angle $\beta$ is, $\beta=\sin ^{-1}\left(\frac{d_{L}-d_{s}}{2 C}\right)$

  Length of cross belt, $L_{c}=\frac{\pi}{2}\left(d_{L}+d_{s}\right)+2 C+\frac{1}{4 C}\left(d_{L}+d_{s}\right)^{2}$

• 1.3) Belt drive with idler pulleys

  A belt drive with an idler pulley (also known as jockey pulley drive) is used with shafts arranged parallel and when an open belt drive can't be used due to the small angle of contact on the smaller pulley. This type of drive is provided to obtain high velocity ratio and when the required belt tension can't be obtained by other means (shown in the figure below).

  

  When it is desired to transmit motion from one shaft to several shafts, all arranged in parallel, a belt drive with many idler pulleys.

2] Compound belt drive

A compound belt drive is used when power is transmitted from one shaft to another through a number of pulleys as shown in the figure below.

• 2.1) Crowing of Belt Pulley

  Before V-belts were invented, machinery was usually powered through flat canvas belts running on crowned pulleys.

  These flat belts stayed centered on pulleys without any guides or flanges. The key to keeping them tracking centered on the pulleys is the use of "crowned pulleys". A crowned pulley is a pulley that has a slight hump in the middle, tapering off ever so slightly towards either edge (shown in the figure below).

  

  How a crowned pulley keeps the belt tracking on it is a mystery to most people, so I thought I'd write a small article explaining how it actually works.

  The key to understanding flat belt tracking on a crowned pulley is to look at how a belt flexes when pulled more on one edge than another. I'm pulling the rubber band in the photo on just the right edge. With more tension on the right side, that side stretches more, and the rubber band forms a slight arch. In an actual belt, this stretch is too subtle to be seen just by looking at it.