Basic Pulley Mechanisms

From tank treads to bike gears to fishing lines, pulleys are used all over the place when it comes to mechanical transmissions. All types of pulley mechanisms consist of some sort of flexible belt (chain, cable, rope, etc.) turning around the circumference of a wheel, and pulleys can be incredibly useful in a variety of situations. In this Instructable I'll go over some basic pulley concepts and interesting mechanisms, and hopefully you'll be able to design your own pulley systems and make stuff like this!

The pulley is one of the six simple machines. Basically, all that a pulley is is a wheel spinning around an axle to aid the motion of a belt. The sprockets on a bike, for example, are a type of pulley, because when they spin, they drive other sprockets on the bike, that in turn rotate the rear wheel. So to make a basic pulley, all you need to do is loop a rope over some sort of wheel and axle.

Two pulleys can be used to create a simple belt and pulley system, in which a belt is looped between the two pulleys. One pulley is the "driving pulley", and as it spins, it transmits power through the belt either via friction or teeth, thus spinning a "driven pulley". I'll be showing how you can use pulleys to make some pretty insteresting mechanisms later on.

Pulleys are probably most commonly used for lifting heavy loads and transmitting power across axes. Elevators, cranes, and boats all use pulleys because a pulley changes the direction of the applied force on the belt. Because the rope or belt is looped around the circumference of the pulley, the force of the object on one end of the rope can loop around the pulley to the other end. Certain types of pulley systems, like the block and tackle (to be explained later), can actually lessen the applied force needed to lift an object via a system of moving pulleys and lines, which can be very useful in high-load situations.

Pulleys are also one of a few different methods of transmitting rotation from one axis to another. Belts and pulleys can be used to transmit power over larger distances and in constricted spaces, which is one advantage they have over gears. Because most pulleys are driven by the friction between the pulley and the belt, if a part of a mechanism jams, then the belt transmitting power will slip on its pulleys instead of stalling the motor.

The Speed Ratio is the ratio of angular velocity of the input pulley of a system to the angular velocity of the output pulley. If you've calculated gear ratios, it is almost exactly the same! This is all based on a pulley's reference diameter, as defined below:

Reference/Pitch Diameter: The working diameter of the pulley, where the belt or cable contacts the pulley. This is what you will use to calculate the speed ratio

The speed ratio is equivalent to the reference diameter of the output pulley over the reference diameter of the input pulley in a two pulley system. Calculating the speed ratio of a more complicated pulley system is fairly simple as long as you take it step by step. With multiple pulleys, the ratio for each segment of the mechanism has to be calculated to determine the overall ratio. In the image above, the lower, driving pulley has a reference diameter of 20 mm and the upper pulley has a radius of 40 mm, making the ratio 2:1. It takes 2 rotations of the lower wheel to rotate the upper wheel once. The speed ratio also tells us something about the torque of the system, as the ratio of the output torque over the input torque is equal to the speed ratio. The upper wheel thus exerts twice the torque, but half the speed.

There are a few different types of pulleys that I'll explain, which apply to multiple types of pulley systems. These the naming conventions for the basic types of pulleys. In future steps you'll see how some of them can be applied to improve mechanisms.

Driving Pulley: The "input" pulley of a two-pulley system. This is the pulley whose shaft is being driven by something, like a motor, crank, or possibly by another pulley if in a larger system. This pulley is what is controlling the motion of the belt.

Driven Pulley: The "output" pulley of a two-pulley system. This is the pulley that is being turned because of the belt's motion.

Idler Pulley: A type of driven pulley not meant to transmit power through its shaft. It spins freely, while most driven pulleys are linked to other devices, like wheels or actuators, via their drive shafts.

Fixed Pulley: A pulley whose axis is fixed in place. It can rotate, but it cannot translate in any direction freely.

Movable Pulley: A pulley that can both rotate and translate based on the motions of the pulley or belt. This is commonly used in block and tackle systems, which I'll explain later.

Guide Pulley: A smaller type of idler pulley usually used to guide cable and keep it along a specific path.

Drum: Common pulleys have a single groove for belts,chains, or cable. Drums are much wider pulleys, usually used in cable systems, that allow the cable to wrap around the drum's diameter multiple times.

Sprocket: A sprocket is used to drive chain systems. It has teeth around its diameter to catch the links in the chain and drive it forward.

A belt and pulley system is one of the simplest types of pulley systems. As I described before, it contains two pulleys, one driving the belt and one driven by a belt. Belt drives can take many different forms; in tank treads, band saws, and sewing machines. Below are the four most common types of belts.

Round Belt: Round belts have a round cross sectional profile. They are used for lighter loads and are usually made of rubber. All "sides" of the profile of the belt are the same, so you can make some fancy pulley systems that interface with different sides of the belt to transmit motion in interesting ways.

Flat Belt: Flat belts have a rectangular cross sectional profile. Usually they are elastic, so they reduce vibration of the belt and usually do not need tensioners as a result.

Timing Belt: Timing belts are like flat belts except they are toothed on their inside face. This allows for more precise control over the position of a mechanism, and it means that power is transmitted via the teeth instead of friction between the belt and pulley. As a result, timing belts don't slip like other belts do, so the pulleys remain in sync. Some mechanisms, like XY gantries, use timing belts and mount parts to the belts to control their position.

V-Belt: V-belts are the most common type of belt. They have a "V" or trapezoidal shaped cross section, which corresponds to the shape of the pulley they rest in. V belts cannot slip out of their pulleys like some of the other belts because of their cross section.

Other types of belts are specific to certain situations but branch off from these four belt types. The blade on a band saw, for example, is a type of flat belt, while tank treads are a type of timing belt.

Although we often refer to the sprockets on a bike as gears, chains mechanisms, like the ones on your bike, are actually pulley systems. The sprockets are just toothed pulleys, and each tooth catches in the link of a chain to pull the chain along. Here are some of the important things to know about chains:

Link: A single unit of chain, consisting of a pin going through two symmetrical plates with one hole for the link's pin and one to cover the pin on the next chain link.

Master Link: A specific clip-like link in a chain that is easier to remove than the other links,so that chain can be easily replaced or tightened without breaking or damaging it.

Chain Breaker: A tool used to push the pins out of a link if links need to be removed to make the loop tighter.

Pitch: The distance between two links of chain. The pitch of a chain and the sprockets it wraps should match.

Teeth: The bumps around the rim of the sprocket that interface with the chain

Reference/Pitch Diameter: The working diameter of the sprocket. This is what you would use to calculate speed ratios. The pitch diameter is equal to the number of teeth times the pitch.

Chains, unlike belts, cannot slip because the teeth mesh with the links in the chain. As a result, they are great for high torque situations, which is why chains are used for things like bikes, motorcycles, and heavy machinery.

A cable drive is a bit different than a belt or chain drive because the cable doesn't have to be a continuous loop. The cable can be fixed at one end and free or attached to something else at the other. A fishing line is a great simple example of a cable system. The line is wrapped around a drum, and by spinning the pulley one way, you can let out the line, and spinning the pulley the other way, you can reel it in. Other things that use cable drive systems include cranes and some weightlifting machines.

Cable drives can be beneficial over belt or chain systems because they don't need a continuous loop to operate, and the cable can be attached to things other than more pulleys. For example, a crane uses cable to pull in and let out the hook block it uses to lift loads. While belts and chain are usually the best for continuous rotational motion of two pulleys, cable drives can be useful because they can be used to manipulate the motion of mechanisms with smaller, slower movements, and these rotational motions can be easily translated to linear movements.

There are lots and lots of different types of pulley mechanisms out there, and this Instructable definitely doesn't cover all of them. However, I hope this will give you a basic idea of some of the ways that you can use pulleys to improve your mechanical design techniques. I'll be starting with some of the simplest mechanisms and design techniques and I'll introduce some more complicated mechanisms, but not all of them. If you're really interested in learning more, I would suggest you check out this book, 507 Mechanical Movements, as it comes with a lot of really neat mechanisms!

As I mentioned previously, the simple pulley is a wheel and axle with a rope or belt looped around it. As the rope is tugged, the pulley turns. The force on the rope travels around the pulley to the other end of the rope, so the pulley changes the direction of the force. If you were to attach a weight to one side of a rope, loop it around a pulley, and pull down on the other side of the pulley, the weight would lift up! It's very simple. The direction of the force of your tug is changed from down to up.

A block and tackle is a cable system primarily found on cranes and boats that involves two pulleys and a cable or rope. The mechanism consists of one fixed pulley and one movable, usually hanging, pulley. A single rope is fixed to on or near the carriage of the fixed pulley, loops down and around the movable pulley, then back up and around the fixed pulley. A hook is attached to the underside of the movable pulley, and by tugging on the rope, the hook will lift or lower.

The beautiful thing about the block and tackle is that it decreases the force required to lift an object. As shown in the image above, the force of the object pulling down is split between the side of the rope with the fixed end and the free end. Thus when you pull on the free end, you only need to exert half the force that you would have had to exert with just a single pulley. The downside of the standard block and tackle is that the weight will only rise half as far as the distance you pull the rope, because the change in distance is split between the rope segment on the fixed end and the free end.

Block and tackle mechanisms can be stacked to incorporate multiple pulleys, which even further decreases the applied load required to lift a weight. Cable mechanisms in cranes frequently use this system to lift very heavy loads.

As I mentioned before, the belt and pulley system is a very simple one, composed of two pulleys and a belt connecting them. You can use a belt and pulley to trasmit rotation from one axis to another by simply spinning one of the shafts connected to a pulley.

A belt and pulley system can transmit rotation and power to other axes over long distances and tight spaces. To do this with something like a gear mechanism, you either need a lot of gears, or very large gears, and that can ramp up the cost of a product pretty quickly. Another advantage of a belt and pulley system over a gear system is that the direction of rotation is conserved on a standard belt and pulley drive (although it can be altered). If the drive pulley spins a certain direction, the driven pulley will too. This is a big difference from gear mechanisms, in which two adjacent gears will turn in opposite directions.

Most pulley systems are friction based, which means that if the one side of a belt and pulley system jams,the belt can slip against the pulley if it needs to. Although this may sound bad, it is actually beneficial because it prevents the system from stalling out the motor by taking on too much torque. Band saws are a great example of this. The blade of the band saw is a large loop that acts as the belt, and two large pulleys turn the band saw to make it cut. If the blade were to catch badly on something, the saw would simply jam while the motor would keep turning the drive pulley.

Winches are mechanisms that allow you to wind up or unwind cable. They provide the basis for many large cable drive systems, because they consist of a large drum that can spool up the cable and can be used to collect slack. Many winches come with a ratcheting system that stops the drum from spinning if the cable is tugged on, which can be very useful in heavy lifting machines like cranes. A simple example of a winch is a fishing rod. When you cast out a rod, the ratchet is released and the line can extend freely. Once the ratchet is locked in place, any tug on the line will not turn the drum, but you can reel the line in to shorten it. A winch is a very simple, yet powerful cable drive mechanism.

While most simple pulley systems usually just spin in one direction, you may need to reverse the direction of spin on one of your axes. These methods cannot be applied to all types of pulley systems as they depend on the flexibility and type of belt. Here are two of the most common ways to invert rotation:

Cross Belt Drive: The simplest method to invert rotation is by "flipping" one side of your belt so that the belt loop creates a figure 8. This is commonly seen in cable drive mechanisms. However, this technique cannot be applied to chain or belts with specific profiles, like V-belts or timing belts, because chains are not flexible enough, and the pulley would interface with the outside face of the belt. This technique may be tricky if the pulleys are close together.

Outside Pulley: A method common to chain driven mechanisms involves placing an idler sprocket on the outside of the chain loop. This can be done with belts too, except the pulley will be interfacing with the outside of the belt if this were to happen.

One of the great things about pulley systems is that they can be very modular, and you can make very simple mechanisms to create variable speeds and torques in a system. Here are a few different ways you can do this:

Speed Pulleys: This is pretty common in drill presses and lathes. By stacking pulleys of different diameters on top of one another, you can create different speed options just by sliding the belt onto a different set of pulleys. Each set of pulleys is paired such that the belt length and the distance between the pulleys' pivots stay the same while the speed ratio changes. This is similar to how a bike chain mechanism works, except on a bike a tensioner (which I'll talk about soon) compensates for the slack on the chain.

Cone Pulleys: Cone shaped pulleys can be used to manipulate the speed as well. This system gives the user much more fine control over the speed ratio of the system, and is commonly used on milling machines and some other rotary tools. While stacking the pulleys allows for set ratios that need to be changed while the machine is stopped, with conical pulleys the belt can and should be moved while the mechanism is running, as the belt makes use of the spinning pulleys to slide up or down the pulleys. Cone pulleys usually work best with flat or round belts.

Tensioning pulleys is a pretty important aspect of pulley design. Most belts, including chain, may not be a perfect fit on the system you design, and may stretch a little after repeated use. This is when tensioners come into play. Tensioners are usually idler pulleys or sprockets whose position can be adjusted. They can either be found on the outside of the belt pushing inward, or on the inside of the belt pushing out. Belts can be tensioned in one of two ways:

Manual Tensioning: The tensioning pulley is attached to some sort of locking slide system so that a user can push the pulley tighter against the belt to maintain tension.

Self Adjusting Tensioning: The tensioning pulley is attached to some sort of spring or weight so that there is constantly a force pushing the pulley up against the belt.

Note: You may not need or want to tension your belts all the time. If your mechanism isn't going to spin at very high speeds, the pulleys are not very far apart, or your belt is elastic and thus self tensioning, you may not need to use a tensioning system on your mechanism. However, it's never a bad idea.

Using some of the ideas from the mechanisms used to invert the rotation of the pulleys, we can transmit rotation from one axis to a perpendicular axis. Like the rotational inversion mechanisms, chain cannot be used on either of these mechanisms because chain isn't flexible in the sideways direction.

To An Axis on the Same Plane: If you want to use a pulley system to transmit rotation from a driving axis to another axis on the same plane, you'll need to feed the belt around two idler pulleys and onto the pulley you want to drive. The idler pulleys basically allow the belt to "bend" at a given angle to get to the driven pulley on the other side. This system can work with most belts and cable, although it will not work with timing belt pulleys because they aren't very thick, and the idlers contact the side of the belt.

To An Axis on a different plane: To transmit rotation to an axis on a different plane, all you have to do is twist the belt! This requires wider pulleys, to give the belt space to wrap around, as the belt may come and leave contact with the pulley at an angle. This means that timing belts are unsuitable for use on this type of mechanism, because they require meshing with the teeth on timing belt pulleys.

Now its your turn to make something cool with pulleys! I made this simple 3D printed PulleyBot to go along with this Instructable, but there are many other directions to go in from here. Use what you've learned and don't forget to share it!

If you have some more pulley wisdom to share, have made any pulley mechanisms or have any questions about mechanisms, please do so in the comments!