Profiled linear guides are typically used in conjunction with a drive system, such as a belt or ball screw, to provide rigid, accurate motion. When the specification calls for extremely long travel and high thrust force, the drive mechanism of choice is commonly a rack & pinion system. Fortunately for design engineers and machine builders, several linear guide manufacturers offer profiled rail guides with integrated rack & pinion assemblies. Integrating the drive and guide functions into one mechanism gives several advantages, not the least of which is ease of assembly. But integrated profiled rail rack & pinion systems offer other benefits as well.
Image credit: Bosch Rexroth Corporation
Design and benefits
Linear guides incorporate gear racks in one of two ways: with the gear teeth machined directly onto the profiled rail, or with a gear rack designed to allow mounting of the rail directly on the rack. While the integrated design is more compact and provides the easiest assembly, the benefit of having the gear rack and profiled rail provided separately is that the components can be replaced independently if one is damaged. This lowers repair time and cost and simplifies replacement part stocking.
Image credit: THK Co., Ltd.
The majority of profiled rail rack & pinion assemblies use helical teeth, which operate with less noise and have a higher load capacity. Some manufacturers do, however, offer straight cut gears, which require less lubrication and are simpler to install. Regardless of tooth design, virtually all profiled rail rack & pinion assemblies are manufactured with a pressure angle of 20 degrees. The pressure angle, as explained by gear manufacturer Boston Gear, is “the angle at a pitch point between the line of pressure which is normal to the tooth surface, and the plane tangent to the pitch surface.” The pressure angle is significant because it influences the load carrying capacity, noise, and backlash behavior of the gear. The 20 degree pressure angle is chosen for its high load capacity.
Image credit: Boston Gear
Sizing and selection considerations
Like other drive units, a key element of sizing a rack & pinion system is to check that the transmitted torque doesn’t exceed the maximum allowable torque, which is based on the pinion design, rack hardness and strength, and tooth pitch. Also important to consider is the pitch deviation, which affects positioning accuracy.
Image credit: Schneeberger AG
In addition to a compact design and simple assembly, another significant benefit of integrated profiled rail rack & pinion devices is the ability to achieve virtually unlimited travel lengths. Both profiled rails and gear racks can have multiple sections that are precision-joined, so joining lengths of integrated rail and rack is an effective and relatively simple way to achieve long travel lengths. In addition, multiple pinion gears can be used on the same rack, to provide independent movement of several workpieces, robots, or tools.
Rack & pinions are known for their ability to withstand moderate debris and liquids, but their open design does make them susceptible to contamination, so mounting orientation (i.e. mounting the assembly on its side or upside down) should be considered for extremely harsh environments. And, in many applications, the assembly can be mounted above the working area, which limits exposure to debris and liquids.
The most common applications for integrated profiled rail and gear racks involve material handling or transport over long lengths. One example is positioning multiple welding robots independently along an automotive assembly line. Other examples include packaging equipment and machining centers with extremely long strokes.
Feature image credit: Schneeberger AG
To Danielle Collins.
I have been researching rack and pinion actuators.
I have a particular application to achieve, just some ideas and advice would be extremely helpful.
Imagine a Utility, what in New Zealand we call a Ute, in The States maybe a truck,, i.e an El Camino
That ute is going to have a removable roof. Basically the removable part is to top rectangle, with a triangle downwards wall on each side, the back window will fold down first, then that roof will slide backwards, down the walls of the rear tray, so two rack and pinion actuators would be required to slide it backwards.
But, then to send the roof in a downwards motion, the two triangle walls would need to fold under and inwards to the rectangle roof, with rack and pinion actuators equally pulling the bottom of the triangle walls down NOT at 90 degrees, but at an equal – 60 (or 120) degree angles.
The result is that the top roof is now down in the ute’s tray, level with the top of the wellside walls.
To see the same project on a similar Ute, (mine is an HZ, this is an HX), please copy and paste this link:, apologies it is so long
http://www.justauto.com.au/justcars/news-and-reviews/holden-hx-custom-roadster-project-1adc70ec-7a80-4ad3-88a8-4efb9de20bfd?backurl=http%3A%2F%2Fwww.justauto.com.au%2Fjustcars%2Fnews-and-reviews%2F%3Fsort%3DOldest%2Blisting%26current_page%3D90&backtext=Results
Except the modifier of the HX had no automatic removable roof, where as mine will..
Danielle, I can understand how to pull the triangle walls backwards, but when they are as far back as need to be…. how do you suggest the bottom of the triangle walls then effectively change a 90 degree direction, then that process reversible to raise the roof and then send it forward to click into it’s usual place.
Hope that sort of makes sense, it is quite hard to explain without diagrams.
Thanks kindly
Ted Charlton
New Zealand
catrax.nz@hotmail.com