Rack and Pinion Drives: Overview
You might see rack and pinion technology on a rack railway, where the rotation of a pinion mounted on a train car engages a rack to get the train up a steep mountain. Rack and pinion technology is also often used in actuators and in pipeline transport, helping the valves of a water pipe to turn. You’ll likely see rack and pinions used in elevators in gates, where safety is critical.
Many engineers think rack and pinion technology is old, cumbersome, and outdated. But in this article, we take a closer look at how the simplicity of rack and pinion drive system design and technology can be ideal for many applications. We also compare rack and pinion drives with linear drives, including belts and ball screws. Read on to learn more!
Rack and Pinion Drive System Design
A rack and pinion is a type of linear actuator made of two parts: (1) the pinion, which is a circular gear, and (2) the rack, which is a linear gear. The pinion engages the rack to effectively translate a rotational motion into a linear motion. For example, if you drive the pinion into rotation, the rack will be driven linearly, and the converse is also true; if you drive the rack linearly, the pinion will be driven into rotation.
Rack and pinion drives may use either straight gears or helical gears. The helical option can be less expensive, but it also increases side torque and operating temperature, a factor that often leads to early signs of wear.
The straight option, on the other hand, operates at lower driving force and a lower operating temperature, in turn using less energy and producing less friction, decreasing wear overtime as compared to the helical option. Of course, the amount of force transmitted by any rack and pinion drive depends on factors such as the tooth pitch, gear ratio, and the size of the pinion.
Rack and Pinions vs Belt Drives & Ball and Screw Technology
Hydraulic systems, linear motors, screw drives – you can go in several possible directions when choosing an option for actuation in a linear motor system. With so many options, it may feel like one size can’t possibly fit all uses. Enter (or re-enter) the rack and pinion drive, perhaps the most flexible and widely applicable linear motion solution. In fact, rack and pinion technology is capable of achieving the same level of positioning accuracy and dynamic moves as advanced ball screw drives and linear motions.
When a linear motion system requires accuracy, dynamics, and high thrust forces, or high dynamics, an engineer might consider several mechanical systems to drive the motion. These typically include belt and pulley, ball screw, or rack and pinion technologies. While a belt drive certainly produces high-speed linear motion, you won’t have the highest thrust force or stiffness if you go with this option.
If you select a ball screw mechanism, you’ll gain high thrust force and positioning accuracy, but you’ll sacrifice speed. Rack and pinion systems, however, can give you everything you’re looking for: high thrust force, dynamics, and positioning accuracy: all in one option.
Travel Length
Engineers love the ability a rack and pinion drive has to produce long stroke lengths. When long travel is a necessity and obtaining the right amount of tension on a toothed belt drive is tricky, rack and pinion technology save the day. Ball and screw drives, on the other hand, fall short in this arena. With a ball and screw drive, the screw’s critical speed is only proportional to its diameter, and inversely proportional to its length.
When you need long travel as an outcome but can’t sacrifice all speed entirely, you need a very large screw, or you’ll be faced with the ball screw whip dilemma. If you choose to use either a ball screw or a belt drive, the inertial that the load must control only increases with the stroke length. The more inertia the moved load carries, the harder it is to attain dynamic movement.
Or – you can go with rack and pinion technology. The use of a rack and pinion mechanism means that you can obtain almost unlimited travel length just by mounting sections of the rack at full length. No increase in the size of the technology is needed to achieve this.
And what’s more, rack and pinion drives don’t involve an inertia increase as travel length increases. This means you can attain high speed and dynamic movement no matter what travel length you require.
Accuracy
Rack and pinion drives can be just as accurate as ball and screw drives. In fact, helical teeth are known for their extremely low pitch error rate. When you need accuracy, less backlash, great repeatability rates, and have a directional change requirement, go with rack and pinion technology.
Dynamics and Control
Belt drives have no problem with long travel lengths, but if you can’t sacrifice high thrust force, rack and pinion technology is yet again your best bet. Rack and pinion drives also have less inertia than ball screw mechanisms, giving rack and pinions the plus in any precision motion application.
In a rack and pinion mechanism, the motor needs only to overcome the inertia of the pinion, while the rack stays still. Conversely, in a ball and screw mechanism, the motor has to overcome the inertia of the entire nut and screw shaft.
Stiffness
Belt drives are known for their low stiffness, which only gets worse over time. The effect of this means precision control can be very difficult, and over the life of a machine, you’ll have to re-tune and re-tension the belt multiple times.
Rack and pinion mechanisms, on the other hand, have higher stiffness than belt drives, and unlike belt drives, stiffness remains consistent with time. The result? Increased stability and predictability.
Rack and Pinion Drive Still Deliver it All
Everything you want in a single option: unlimited travel length, high speed, dynamic movement, consistent high stiffness, and positioning accuracy. With rack and pinion technology – what’s not to love? If you’ve been ignoring this mechanism, maybe it’s time to take a new look.
