Linear Power Transmission

Rack and Pinion

A rack and pinion is a type of linear actuator that consists of a circular gear (Pinion) engaging a linear gear (Rack), which operate to translate rotational motion into linear motion.

Driving the rack linearly will cause the pinion to be driven into a rotation. Driving the pinion into rotation causes the rack to be driven linearly.  A rack and pinion drive can use both straight and helical gears.

           STRAIGHT GEAR / SPUR GEAR                                      HELICAL GEAR

Helical Gears are preferred due to their quieter operation and higher load bearing capacity. The maximum force that can be transmitted in a rack and pinion mechanism is determined by the tooth pitch and the size of the pinion.

EXAMPLES

RACK AND PINION RAILWAY

In a Rack railway, the rotation of a pinion mounted on a locomotive or a railroad car engages a rack placed between the rails and helps to move the train up a steep gradient.

RACK AND PINION STEERING 

A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack. The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle. 

Power Rack-and-pinion

When the rack-and-pinion is in a power-steering system, the rack has a slightly different design.

Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist.

Rack-and-pinion Actuators

Actuators use rack and pinion with two racks and one pinion.

Lead Screws

Lead screws act as a linkage to covert rotational motion into linear motion. Although used in a wide variety of applications, they are not efficient enough due to the large frictional energy losses.

Advantages: They have large load carrying capacity and mechanical advantage, low cost, simple to design and manufacture, and self locking capabilities.

Factors to consider while selecting lead screws: load capacity, pressure-velocity factor, efficiency, speed, duty cycle ,backlash ,life and lubrication.

Ball Screws

With a little friction, the ball screws convert linear motion into rotational motion. However due to its lower friction, ball screws tend to back drive under higher load and are expensive.

Advantages: More efficient, runs at cool temperatures.

Preload is the tension created in a fastener when it is tightened. Connections using preloaded bolts are designed to transfer load without any slip.

Standard Lead  Precision Ball Screw				Standard Lead Rolled Ball Screw
Standard Type equipped with Shaft		Standard Nut Type		Standard Nut Type	Rolled Shaft and Nut Standard off-the-shelf Type
Shaft end unfinished type  Models MDK, MBF, BNF, BIF and BNFN	Shaft end finished type Model BNK	Preload type Models SBN, DIK, DKN, BIF and BNFN	Non-preload type Models HBN, DK, BNF and BNT	Preload type Model JPF	Non-preload type Models MTF, BTK and BNT
BIF		SBN			BTK
MDK & MBF		BIF			MTF
BNF elm		DIK elm			BNT
		DKN elm
		BNFN

Rotary Nut Series
Rotary Nut Precision Ball Screw		Rotary Nut Rolled Ball Screw	Ball Screws/Splines
Standard-lead, preload type Model DIR	Large-lead, non-preload type Model BLR	Large-lead, non-preload type Model BLR	Stroke, rotation type Models BNS and BNS-A	Stroke type Models NS and NS-A
			BNS	NS
			BNS-A	NSA

References

[1] “Trapezoidal thread form,” Wikipedia, 10-Feb-2021. [Online]. Available: https://en.wikipedia.org/wiki/Trapezoidal_thread_form. [Accessed: 04-Mar-2021].

[2] “Buttress thread,” Wikipedia, 11-Feb-2021. [Online]. Available: https://en.wikipedia.org/wiki/Buttress_thread. [Accessed: 04-Mar-2021].

[3] “Types of Ball Screws,” THK, 11-Feb-2021. [Online]. Available: https://pdf4pro.com/cdn/types-of-ball-screws-cbm-ind-43a263.pdf [Accessed: 10-Jun-2021].

[4] “Ball Screws,” THK, 11-Feb-2021. [Online]. Available: https://tech.thk.com/en/products/pdf_download.php?file=511E_15_BallScrew.pdf [Accessed: 10-Jun-2021].

[5] “Rack and pinion,” Wikipedia, 19-May-2021. [Online]. Available: https://en.wikipedia.org/wiki/Rack_and_pinion [Accessed: 10-Jul-2021].

[6] “Rack and pinion steering: Everything you need to know", "Hearst Autos Research", https://www.caranddriver.com/research/a31267607/rack-and-pinion-steering/ [Accessed: 10-Jul-2021].