Linear Power Transmission

Table of Contents

 

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.


Rack and Pinion diagram


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 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 Railway model


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


Rack and Pinion Steering modal

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.

Power rack and pinion steering model


Rack-and-pinion Actuators

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

Rack and Pinion Actuators

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.


Figure 1. Thread Forms/Standards for Linear Power Transmission Tools
Thread FormImage [6]DescriptionUsageStandard

Trapezoidal

Image of a trapezoidal thread form.

  • Trapezoidal outlines
  • High strength and ease of manufacturing
  • 30° flank angle
  • Used for leadscrews (power screws)
  • Used where large loads are required (vice, lathe)

British and American Standard General Purpose ACME

Image of a British and American Standard general purpose ACME thread form.

  • Thread height is half the pitch length and flattened
  • 29° flank angle
  • Used for leadscrews (power screws)
  • Used where large loads are required (vice, lathe)
  • ASME B1.5-1997 (R2014)

American Standard STUB ACME

Image of an American Standard STUB ACME thread form.

  • Same as ACME except different height [1]
  • Used for leadscrews (power screws)
  • Used where large loads are required (vice, lathe)
  • ASME B1.5-1997 (R2014)

BUTTRESS (Continental)

Image of a BUTTRESS (Continental) thread form.

  • Asymmetric thread form
  • Allows for low friction, withstands greater loads in one direction, but higher friction and inferior load bearing in the opposite
  • One side at a slant typically less than 7°, while the other is at an angle typically at 45°
  • Continental BUTTRESS thread form has 45°/5° 
  • Used in the construction of artillery
  • Used in jacks, lifts, hydraulic sealing
  • Applications in automotive to water and sewer
  • ANSI/ASME B1.9-1973 (R2017)

British Standard BUTTRESS

Image of a British Standard BUTTRESS thread form.

  • 45°/7°
  • Used in the construction of artillery
  • Used in jacks, lifts, hydraulic sealing
  • Applications in automotive to water and sewer

American National BUTTRESS

Image of an American National BUTTRESS thread form.

  • 45°/7°, but internal threads are flat [2]
  • Used in the construction of artillery
  • Used in jacks, lifts, hydraulic sealing
  • Applications in automotive to water and sewer

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 ScrewStandard Lead Rolled Ball Screw
Standard Type equipped with ShaftStandard Nut TypeStandard Nut TypeRolled 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


Large-Lead Precision Ball ScrewLarge-Lead Rolled
Ball Screw

Standard Nut Type
Rolled Shaft and Nut
Standard off-the-shelf Type

Preload type

Models SBK and BLW

Non-preload type
Models BLK and WGF

Non-preload type
Models BLK, WTF and CNF

BLW

BLK & WGF

BLK/WTF

SBK

CNF

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].