Type of linear actuator
Animation of a rack and pinion
A rack and pinion is a type of linear actuator that comprises a circular gear (the pinion) engaging a linear gear (the rack). Together, they convert between rotational motion and linear motion. Rotating the pinion causes the rack to be driven in a line. Conversely, moving the rack linearly will cause the pinion to rotate. A rack-and-pinion drive can use both straight and helical gears. Though some suggest helical gears are quieter in operation, no hard evidence supports this theory. Helical racks, while being more affordable, have proven to increase side torque on the datums, increasing operating temperature leading to premature wear. Straight racks require a lower driving force and offer increased torque and speed per fraction of gear ratio which allows lower operating temperature and lessens viscal friction and energy use. The maximum force that can be transmitted in a rack-and-pinion mechanism is determined by the torque on the pinion and its size, or, conversely, by the force on the rack and the size of the pinion.
For example, in a rack railway, the rotation of a pinion mounted on a locomotive or a railroad car engages a rack, usually placed between the rails, and helps to move the train up a steep gradient.
For every pair of conjugate involute profile, there is a basic rack. This basic rack is the profile of the conjugate gear of infinite pitch radius (i.e. a toothed straight edge).[1]
A generating rack is a rack outline used to indicate tooth details and dimensions for the design of a generating tool, such as a hob or a gear shaper cutter.[1]
Applications
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Rack-and-pinion combinations are often used as part of a simple linear actuator, where the rotation of a shaft powered by hand or by a motor is converted to linear motion.
The rack carries the full load of the actuator directly and so the driving pinion is usually small, so that the gear ratio reduces the torque required. This force, thus torque, may still be substantial and so it is common for there to be a reduction gear immediately before this by either a gear or worm gear reduction. Rack gears have a higher ratio, thus require a greater driving torque, than screw actuators.
Stairlifts
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Most stairlifts today operate using the rack-and-pinion system.[citation needed]
Steering
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Rack steering in an automobile
A rack and pinion is commonly found in the steering mechanism of cars or other wheeled, steered vehicles. Rack and pinion provides less mechanical advantage than other mechanisms such as recirculating ball, but less backlash and greater feedback, or steering "feel". The mechanism may be power-assisted, usually by hydraulic or electrical means.
The use of a variable rack (still using a normal pinion) was invented by Arthur Ernest Bishop[2] in the 1970s, so as to improve vehicle response and steering "feel", especially at high speeds. He also created a low cost press forging process to manufacture the racks, eliminating the need to machine the gear teeth.
Rack railways
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Rack railway axle (wheelset)
Rack railways are mountain railways that use a rack built into the center of the track and a pinion on their locomotives. This allows them to work on steep gradients, up to 45 degrees, as opposed to conventional railways which rely on friction alone for locomotion. Additionally, the rack and pinion addition provides these trains with controlled brakes and reduces the effects of snow or ice on the rails.
Actuators
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Pneumatic rack-and-pinion actuators
A rack and pinion with two racks and one pinion is used in actuators. An example is pneumatic rack-and-pinion actuators that can be used to control valves in pipeline transport. The actuators in the picture on the right are used to control the valves of large water pipeline. In the top actuator, a gray control signal line can be seen connecting to a solenoid valve (the small black box attached to the back of the top actuator), which is used as the pilot for the actuator. The solenoid valve controls the air pressure coming from the input air line (the small green tube). The output air from the solenoid valve is fed to the chamber in the middle of the actuator, increasing the pressure. The pressure in the actuator's chamber pushes the pistons away. While the pistons are moving apart from each other, the attached racks are also moved along the pistons in the opposite directions of the two racks. The two racks are meshed to a pinion at the direct opposite teeth of the pinion. When the two racks move, the pinion is turned, causing the attached main valve of the water pipe to turn.[3][4]
Arcuate rack
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A rack gear that is curved is called an arcuate rack.[5]
History
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The rack-and-pinion mechanism was first developed in China by firearms designer Zhao Shizhen. In his book of 1598 AD, the Shen Qi Pu (神器譜), the Xuanyuan arquebus (軒轅銃) featured a firing mechanism using a rack-and-pinion system that was inspired by Turkish matchlock designs that featured a novel pivoting firing mechanism. The Xuanyuan arquebus was designed in response to contemporary firearm unreliability problems arising from rainy and windy conditions, and offered a trigger that simultaneously operated both the flash pan and serpentine. The Wu Pei Chih (1621) later described Ottoman Turkish muskets that used a rack-and-pinion mechanism. [6][7]
See also
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References
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There are many mechanisms that convert rotary motion of an electric motor to linear motion, such as belt/chain drives, screw drives, rack & pinion drives and even CAM drives. Each mechanism offers advantages and disadvantages. Choosing the right technology can help increase load, speed, travel distance, or positioning accuracy.
Belt & Pulley - recognized for their long travel and high speeds, belt drives have low initial cost and are popular for general purpose applications. However, after a certain length, tensioning of the belt becomes an issue. Its typical polyurethane belts are usually reinforced with steel, and the lack of rolling elements helps withstand harsh environments. The drawback of belts is that they are elastic and can stretch or wear over time. Therefore, belt drives are not recommended for high precision or vertical applications with high acceleration rates or heavy loads. The elasticity of belts also increase resonance and settling time more than other mechanisms. For light loads traveling horizontally, belts typically are sufficient.
Lead/Ball Screw - combined with either a lead screw or a more efficient ball screw with a fine pitch/lead, screw drives can achieve high thrust forces and greater positioning accuracy and repeatability than other mechanisms, but at a higher cost and lower speed. When the travel distance is increased by extending the length of the screw, the screw's whipping effect decreases the max permissible speed. Additional screw support structure can reduce the whipping effect, but it increases both the cost and footprint. The inefficiency of the lead screw can be used as an advantage to provide holding force for vertical loads, while the high efficiency of ball screws allow higher torque conversion to thrust force.
Rack and Pinion - known for their long strokes, rack and pinion drive systems virtually have no limit on travel length as long as the rack can be made long enough. The max travel length is instead limited by its support structure. By using a rack instead of a belt, elasticity is eliminated, and rigidity is increased. However, since a rack is heavier than a belt, some speed is sacrificed. Rack and pinion systems are ideal for applications such as vertical lifts or overhead gantries. Its design also allows multiple carriages to be used on one system, which is useful for applications that require loads to move independently. The difference in the design and machining quality of the racks makes a difference in performance in noise or accuracy.
Here's a quick comparison table for the 3 mechanisms discussed.
So which mechanism is the best? It depends on the job.
Various electric linear motorized slides, cylinders, and actuators are offered with ball screw, belt as well as rack and pinion technology to suit different requirements. For an application that requires a high load but doesn't require the precision of screws, a rack and pinion system makes sense.
Jump to the DEMO VIDEO to see an application example, or if you have some time to burn, read on to let me tell you why Oriental Motor's rack and pinion systems can resolve some typical linear motion mechanism challenges.
Increasing load mass is difficult when ball screws are used.
Heavier loads have more weight and mass. Motorized slides with ball screws typically carries the load on a carriage that moves along the outside of screw; therefore additional moment load considerations may be necessary for heavier loads. Increasing the load from 30 kg to 100 kg with the ball screw example below can increase the moment load, potentially exceeding the specification and thus requiring additional guide support. Rack and pinion systems can reduce the need for external support while increasing transportable mass for vertical applications.
Ball screws and cams are difficult to obtain.
Ball screws are highly advanced mechanisms with rolling elements, and cams are not easy to design. Typically, lead times are long for these parts. It's possible for engineers to build their own, but a significant amount of engineering resources is necessary.
Credit: https://en.wikipedia.org/wiki/Cam
Wiring for sensors and other devices is complicated.
For position control applications, a homing routine is necessary for ensuring the same starting position every time and maintaining accuracy. For a typical motor, 3 sensors are used for home, + limit, and - limit. By using a motor with an built-in multi-rotation absolute encoder, these sensors and the time required for installation and testing can be eliminated. Some new types of absolute encoders do not need a backup battery and can eliminate maintenance requirements associated with them.
Improved equipment performance is desired.
Closed-loop stepper motor technology has advanced since its introduction. With Oriental Motor's patented "ABZO" in the AZ Series stepper motor, multiple rotations of absolute coordinates are are available without the need for a backup battery, and slow homing speeds or missed steps are now history.
The patented improves stepper motor performance by combining the best of both closed-loop and open-loop control. Advanced motion sequence programming from the AZ Series such as looped operations and conditional event triggers helps reduce burden on the PLC and makes motion programming easy even for non-engineers.
We set up a test to compare the homing accuracy between a photo sensor and the ABZO sensor.
Here we show how the L Series rack and pinion systems can help automate a process. The advanced motion sequence programming of the dedicated drivers can help reduce burden on the PLC or HMI.
Watch more .
By combining its best closed loop motor technology with a high capacity, long stroke rack and pinion mechanism in the L Series, Oriental Motor offers a 100 kg max load capacity at up to 1 meter stroke with highly reliable servo-like performance with no tuning. In addition, the motor's patented technology minimizes heat generated and allows high duty cycles for increasing throughput of a machine. Multi-axis and fieldbus network capability are available through various drivers. Combined with its extensive remote status monitor functions, predictive maintenance is also possible.
- 7 to 100 kg max transportable mass
- 100 to 1,000 mm max stroke
- 20 to 500 mm/s speed
- Compact footprint
- Pre-assembly saves time
Oriental Motor offers easy sizing for linear motion systems. Easily pick a part number by selecting your required type, load, speed and stroke.
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