Belt drives

SUBJECT OUTCOME
4.1 Demonstrate an understanding of v-belt drives
 
Application of v-belts
 
A belt is a loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. Belts may be used as a source of motion, to transmit power efficiently or to track relative movement. Belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel.

In a two pulley system, the belt can either drive the pulleys normally in one direction (the same if on parallel shafts), or the belt may be crossed, so that the direction of the driven shaft is reversed (the opposite direction to the driver if on parallel shafts). As a source of motion, a conveyor belt is one application where the belt is adapted to carry a load continuously between two points. The belt drive can also be used to change the speed of rotation, either up or down, by using different sized pulleys.

 
Advantages and disadvantages of v-belt drives over chain and gear drives
Advantages Disadvantages
  • They are simple and economical.
  • They can transmit Power over a considerable distance.
  • They can protect the machine from overloading by slipping of the belt over a pulley.
  • Belt drive can absorb shock and damp vibration.
  • The operation is smooth and silent.
  • They are durable and require very little maintenance
  • Drive is not positive as the belt slip over the. pulleys.
  • Occupies relatively more space.
  • Speed ration can not be maintained because. of slipping of belt.
  • Adjust the tension in the belt is require time to. time.
  • Life is relatively low.
  • They cause heavy loads on shaft and bearing became of there require
  • Can not operate in extreme temperature ranges.
Common and special V-belt types  -
     
Classical V-belt

Classical” V-belts are standardized a height to width ratio of 1:1.6. Tension cords made of steel, armour, polyester or glass are embedded in an elastomer core covered by a top layer. The tension cords run at the level of the nominal width (neutral axis).


To increase friction or wear resistance and to protect the belt from harmful external influences, the V-belt can be covered with a special rubber fabric. This is then referred to as a wrapped V-belt. Such wrapped V-belts are used, for example, in drives for pumps in the chemical industry to convey aggressive media.
 

 

   Wedge belt

Compared to standard V-belts, narrow V-belts have a more favourable height to width ratio of 1:1.2. The greater height (at the same width as a classic V-belt) ensures greater power transmission. Conversely, the belt width can be much smaller with the same power transmission. The associated lower belt mass of the narrow V-belt reduces the centrifugal forces occurring during operation, so that higher belt speeds can be achieved.


 
     
Synchronous belt (Timing belts)

With friction-locking belt types such as flat belts and V-belts, slippage will occur, which reduces the efficiency and control accuracy accordingly. However, this can be prevented by toothed belts, as the teeth attached to the belt surface then transmit the force positively. Slippage cannot take place. Therefore, toothed belts are always used when precise positioning is required. For this reason toothed belts are also called timing belts or synchronous belts.

 

   Banded or concord V-belts

Banded belts are two or more v-belts (ribs) joined together to form a single unit. Banded belts are an excellent solution to problem drives with long centre distances or drives with pulsating and high shock loads where single belts vibrate, turn over or jump off the sheaves.
Cogged belt

The increased belt thickness has an adverse effect on the flexibility. To compensate for this and to be able to use narrow V-belts even with relatively small pulley diameters, they are therefore cogged (so-called narrow cogged V-belts). This increases flexibility even with strong curvatures. This is why narrow V-belts are usually found in the raw edge version.

 

  
 

V-belt drive terms and indicate components from given drawings
 

V-belt drive terms and the functions of components
   
When rotating around the pulleys, the belt is exposed to different loads. The section of the belt in which the belt is strongly pulled towards the driving pulley and is thus exposed to a large tensile load is referred to as the tight side. On the opposite section, the belt moves away from the driving pulley and is slightly relieved by its “pushing” effect. This belt section is called slack side.

Tight side - The section of the belt in which the belt is strongly pulled towards the driving pulley and is thus exposed to a large tensile load is referred to as the tight side. On the opposite section, the belt moves away from the driving pulley and is slightly relieved by its “pushing” effect.

Slack side - The slack side is the side where tension on a belt is low compared to the other side. In a flat belt drive, driver pulley pulls the belt through the bottom side and deliver it on the opposite side (upper side), so the top side is always loose. One of the reasons for doing so is to increase the angle of contact

Tension ration - The tension in the ratio between the tight side T1 and the slack side T2.

Tension ratio = T1/T2 (Newton)

Angle of contact (wrap) - In the following, a single-stage belt drive with two pulleys that are wrapped around by a common belt is considered. The magnitude of this wrap is described by the wrap angle

The larger the wrap angle, the more adhesive surface the belt has and the greater the frictional force or the force that can be transmitted. However, it must be noted that the pulleys of a belt drive are wrapped to different degrees by the belt if the pulleys have different diameters!

The maximum transmittable force is usually limited by the smaller of the two pulleys (usually the driving pulley), as this pulley has a smaller wrap angle compared to the larger driven pulley. In addition, the greater curvature causes greater bending stresses in the belt, which also limit the transmittable belt force.

So called idler pulleys can be used to increase the wrap angle. These are usually placed near the actual pulleys to achieve the greatest possible wrapping effect. If these pulleys are used to tension the belt at the same time, they are also called tensioner pulleys.

Jockey (idler) pulleys- If such pulleys are merely used to deflect the belt, then these are generally referred to as idler pulleys. Idler pulleys are used, for example, in multiple drives in which one driving pulley drives several other pulleys. Idler pulleys can also be used for long belt lengths to reduce belt vibrations. Such pulleys can also take over the function of a guide at the same time, so that the belt does not jump off the pulley. Such pulleys are called guiding pulleys or guide rollers. Guiding pulleys often have protrusions (called flanged pulleys) on the left and right, between which the belt is held in track.

Tension pulleys do not yet have a tensioning effect; certain devices are required to achieve a tensioning effect (also known as tensioning systems). Tensioning devices are available in a wide variety of designs. The tensioning systems shown in the figures above were each designed with a simple spring mechanism. The springs offer the advantage that the belt tension can adapt dynamically to the operating state, e.g. strong load changes.

   
Composition of a V-belt, by means of a sectional drawing  
  • Bandply (Cover) - This is made from woven cotton fabric impregnated with neoprene.
  • Tensile cords - are made of high strength synthetic fibre that can accommodate the belt tension.
  • Over cord (Tension section) - are made of a synthetic rubber compound that is able to stretch as the belt bends around the pulleys.
  • Under cord rubber (Compression section) - are made from a synthetic rubber compound that is able to support the tensile cords while bending around the pulleys

 

   
V-belt pulleys for single and multiple belt drives using a simple drawing  
   
Single belt pulley Multi groove pulley
   
   

Additional websites
Tec-science
Belt-drive/basics
V belt supply