Page Comparison

Fixed Pulley

A fixed pulley is a pulley that is mounted to a stationary surface [3]. If a cord or belt is passed through a fixed pulley the pulley will rotate but not move. The main advantage of fixed pulleys is that they can redirect force [3]. A well uses a fixed pulley because it allows a person to lift a bucket of water by pulling a rope downward.

Moveable Pulley

A moveable pulley is a pulley that is mounted to a moveable surface (typically the load) [3]. If a cord or belt is passed through a moveable pulley, the pulley will rotate and move. The main advantage of moveable pulleys is they lessen the force required to move a load using mechanical advantage [3]. 

Compound Pulley System

A compound pulley system combines fixed and moveable pulleys to take advantage of the benefits of both types [3]. Construction cranes use a compound pulley system to lower the force required to lift construction materials and redirect the force to the other side of the crane where the motors and winch are to help counterbalance the load.

Specialized Pulley Systems

Block and Tackle

A block and tackle is a compound pulley system where there are several fixed pulleys on one axis and several moveable pulleys on another axis [3]. The purpose of a block and tackle system is to drastically reduce the force required to move a load [3]. Boats use block and tackle pulley systems because they allow the sails to be easily adjusted, even in high wind, from a convenient location on the boat. In block and tackle systems the belts are made of rope.

Cone Pulley

A cone pulley is a fixed pulley where there are multiple grooves of different diameters on the same pulley [3]. The purpose of a cone pulley is to change the speed and torque of the output similar to changing gears on a bicycle [3]. Lathes use cone pulleys to adjust the speed of the spindle.

Tensioner Pulleys

A tensioner pulley is a fixed pulley that is used to tension a belt. Tensioner pulleys are mounted to a spring load mechanism or adjustable pivot point that allow them to push on the belt to apply tension and prevent the belt from slipping. Tensioner pulleys are often used in systems that require precision or efficiency such as an engine timing belt. Having a good tensioner in an engine, is very important because if the timing belt loosens and the engine timing is off valves can contact the pistons and destroy the engine [9].

Continuously Variable Transmission (CVT) Pulleys

A CVT uses 2 fixed pulleys with wedge edges for a v-belt. The v-belt is of a specific width so by moving the distance between the wedge faces of a pulley you can make the belt move up or down on the pulleys which changes the gear ratio. The pulleys must change oppositely to each other to keep the v-belt tensioned to the pulleys. The distance between the wedge pulley faces can be changed in very small increments which allows this pulley system to be continuously variable. CVT pulley systems are frequently used in cars because they allow the vehicle to accelerate while keeping the engine rpm low which is very fuel efficient [11].

Idler Pulleys

Idler pulleys are pulleys that will rotate independent of the rotation of the shaft. Idler pulleys typically use bearings the rotate independently of the shaft. There are 2 common ways of mounting idler pulleys. First, use a washer and bolt to hold the center of the bearing in place on a threaded standoff. This method is used to mount engine idler pulleys. Second, interference fit the bearing onto a shaft. This is usually done by heating the center of the bearing and/or cooling the shaft. Thermal expansion and/or contraction will allow the pulley bearing to fit over the shaft and once at room temperature the pulley bearing will be interference fit onto the shaft.

Fixed Pulleys

Fixed pulleys are pulleys that will rotate with the rotation of the shaft. There are 3 common ways of mounting fixed pulleys. First, use a key that goes into a keyway in the pulley and shaft and/or a set screw that goes through the pulley to a flat section on the shaft. Second, use tapered bushings that go between the pulley and shaft. Tapered bushings make use of the wedge effect to convert the clamping force into radial pressure that locks the bushing to the pulley and shaft [12]. Third, interference fit the pulley onto a shaft. This is usually done by heating the pulley and/or cooling the shaft. Thermal expansion and/or contraction will allow the pulley to fit over the shaft and once at room temperature the pulley will be interference fit onto the shaft.

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 can be put on slightly curved surfaces. 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 [15].

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 [15].

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 [15]. 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.

Wrapped, Raw Edge, and Cogged V-Belts

Every type of v-belt is either wrapped or raw edge. Wrapped v-belts have a complete outer layer around an elastomer core with tension wire. Wrapped v-belts offer additional protection over raw edge v-belts, so they are used in harmful environments such as on a hazardous chemical pumps. Raw edge v-belts have a top layer, an embedding layer for the tension cord, and a bottom elastomer core layer. Raw edge v-belts are more flexible than wrapped v-belts, meaning they wear slower and are more efficient. Raw edge v-belts are used when a wrapped v-belt is not necessary. A cogged v-belt is a raw edge v-belt that has pieces of the bottom elastomer core layer removed to allow for greater flexibility. Cogged v-belts look like toothed belts, but they are not because the pulleys do not have teeth that the belt can lock into [15].

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 [15].

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 [15].

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 [15].

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 [15].

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 [15].

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 [15]. 

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 [15].

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 [15].

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