Belts

Flat Belts

A flat belt is the simplest type of belt and it has a rectangular cross section. A typical flat belt will have a protective top layer, a middle layer made of tension cords and a rubber core, and a protective bottom layer often coated in a special rubber compound that increases the friction between the belt and the surface it is on and to increase the life span of the belt. If both sides of the belt are used, such as in a complex pulley system, the top layer of the belt would be coated like the bottom layer. Flat belts are often put on bulging pulleys so that they self-center on the pulley. The bulge is typically 0.3mm to 1.2mm in height. Flat belts are around 98% efficient. The main advantages of flat belts are that they are quiet, low wear, and are around 98% efficient. The main disadvantage is that a lot of force is needed to tension the belt to keep it on the pulley and the high tensioning forces puts a lot of stress on the bearings of the pulley [4].

V-Belts

A V-belt is the most versatile type of belt and it has a wedge cross section. The wedge shape of v-belts takes advantage of the wedge effect which allows v-belts to have low tensioning forces. This means that a v-belt with the same tensioning as a flat belt can hold much more torque. A v-belt is thicker than a flat belt so it has to bend more to travel around a pulley which lowers efficiency. The wedge sides of the v-belt touch the pulley but the bottom of the belt does not touch the pulley. V-belts are designed to be used for specific pulley diameters at specific wrap angles because v-belts buckle and will stretch off of the wedge sides of the pulley, causing the belt to slip. The groove angle for most v-belts is either 32° or 38°. The tensioning cords of a v-belt are placed near the top, at the nominal width. The main advantage of v-belts is that they have low tensioning forces, so the bearing are under significantly less stress. The main disadvantage of v-belts is that they are around 95% efficient [4].

Wedge Effect

V-belts are able to have lower tensioning forces than flat belts because of the wedge effect. For flat belts, the force of friction keeping the belt on the pulley is proportional to the radial force of the belt on the pulley. For v-belts, the force of friction keeping the belt on the pulley is proportional to the sum of the normal forces from the two angled wedge surfaces the belt rides on [4]. Based on the formulas in the image on the right, a v-belt with a groove angle of 32° will produce 3.6 times more friction with the same tensioning force. For equal frictional forces, the v-belt will require 0.28 times the tensioning force of a flat belt, meaning the bearing for a v-belt experiences almost a quarter of the load of a flat belt bearing.

Standard V-Belts

Standard v-belts are v-belts with a height to width ratio of 1:1.6. Standard sized belts balance the flexibility and power transmission of the narrow and wide belts [4].

Narrow V-Belts

Narrow v-belts are v-belts with a height to width ratio of 1:1.2. When compared to a standard v-belt of the same width, the increase in height allows for a better transmission of power at the cost of lower flexibility. A narrow v-belt with the same height as a standard v-belt transmits power as well, but it is thinner allowing it to fit better in compact machines. To offset the loss in flexibility, narrow v-belts can be cogged and/or raw edge [4].

Wide V-Belts

Wide v-belts are v-belts with a height to width ratio of 1:2. When compared to a standard v-belt the increased width means the v-belt is stronger. Wide v-belts are used for heavy power transmission and when large changes in speed occur such as in a continuously variable transmission (CVT). To increase flexibility, most wide v-belts are cogged and raw edge [4].

Double V-Belts

Double v-belts, also known as hex-belts are essentially two v-belts put together so that their cross section is a hexagon. Double v-belts are used when both sides of the v-belt need to transmit power [4].

Kraftbands

Kraftbands are v-belts that have several v-belts connected to a cover plate by the top of each v-belt. The combination of multiple v-belts ensures that individual belts do not jump off the pulley under impact loads [4].

Poly V-Belts

Poly v-belts, also known as serpentine belts, are v-belts that are shaped like kraftbands, but use a single embedding layer with tension cord like a flat belt. The combination of v-belts and flat belts allows poly v-belts to have advantages from both types. The main advantages of poly v-belts are high flexibility, high power transmission, and low bearing load. Poly v-belts are often used when one pulley drives multiple pulleys such as engine serpentine belts [4]. 

Round Belts

Round belts are belts that have a circular cross section. Round belts are often used for motion transition and are very rarely used for power transmission. Round belts can be guided very easily with guiding pulleys due to their round cross section [4].

Timing Belts

Timing belts, also known as synchronous belts, are belts that have teeth that lock into matching teeth on the pulleys. Timing belts prevent slippage, which is why they are used in applications where keeping timing or accuracy is important such as engine timing belts or 3D printer belts [4].

Contributors:

Faculty Advisor: Chris Rennick, Michael Lenover (Alumni)