Rack & Pinion

There are four basic quality levels of gear racks available, each of which is well suited for certain types of applications:

Rack Type:	Tooth Quality:	Tooth Hardness:	Tooth Pitches:	Segment Lengths:	Typical Applications:
Soft Rack	~AGMA 9 (DIN 9)	~28 Rc	Module 1.0 to 10.0	Up to 3.0 Meters	Light Loads Medium Accuracy
Quenched & Tempered Rack	~AGMA 10 (DIN 8)	~28 Rc	Module 2.0 to 5.0	Up to 2.0 Meter	Medium Loads Medium Accuracy
Induction Hardened Rack	~AGMA 8 (DIN 10)	50 - 55 Rc	Module 2.0 to 10.0	Up to 2.0 Meters	Heavy Loads Low Accuracy
Hardened & Ground Rack	~AGMA 12 (DIN 6)	60 Rc	Module 2.0 to 10.0	Up to 2.0 Meters	Heavy Loads High Accuracy

The quality levels denote the accuracy of the teeth, which affects the positioning accuracy, backlash and noise level of the rack & pinion set.  We always recommend a hardened & ground pinion regardless of the rack type used.

Don't let the rack segment lengths above deceive you - unlimited travel lengths can easily be achieved by mounting the racks end-to-end, using a companion rack to properly space each segment.  What is a Module?    Module is a metric pitch, inversely related to diametral pitch, e.g. Module = 25.4 / Diametral Pitch.

The most recent addition to this range is our Integrated Rack & Rail , for mounting directly to linear guides, simplifying alignment and machining.

Selecting The Right Rack:

Selecting the right gear rack depends on many things, including accuracy and load requirements, design constraints, and cost.  Sometimes it is possible that two different racks would work for the same application, but each having it's own advantages and disadvantages.

Rack & pinions are typically selected based on the peak cycle forces, which usually happen during accelerations and decelerations.  These cycle forces depend on the weight being moved, the speed vs. time profile for the cycle, the friction of the axis bearings, and any other external forces acting on the axis.

Maximum tangential force ratings for the various racks can be seen in the following table:

Rack & Pinion Size	Maximum Tangential Force Ratings *
	Hardened & Ground Rack	Induction-Hardened Rack	Quenched & Tempered Rack Soft Rack
Module 2.0	1,900 lb.	1,500 lb.	up to 1,000 lb.
Module 3.0	3,400 lb.	2,800 lb.	up to 2,300 lb.
Module 4.0	6,500 lb.	5,300 lb.	up to 4,300 lb.
Module 5.0	7,700 lb.	7,000 lb.	up to 4,200 lb.
Module 6.0	11,000 lb.	10,000 lb.	up to 4,400 lb.
Module 8.0	21,000 lb.	16,000 lb.	up to 7,600 lb.
Module 10.0	31,000 lb.	22,500 lb.	9,000 lb.

Helical versus Straight .  There are two styles of gear racks we offer - Straight tooth, where the teeth are perpendicular to rack length, and Helical tooth, where the teeth are at an angle to the rack length.  The helical style provides several key benefits over the straight style, including:

• Helical rack & pinions runs quieter than the straight, especially at high speeds

• Helical rack & pinions has a higher contact ratio (the number of effective teeth engaged) than straight, which increases the load carrying capacity

• Helical rack lengths are nice round numbers, e.g. 500.0 mm and 1,000.0 mm, for easy integration with machine bed lengths; Straight racks lengths are always a multiple of pi., e.g. 502.65 mm and 1005.31 mm.

• In most cases, helical rack & pinions cost the same as the straight rack & pinions!

The only real disadvantage to using helical rack & pinions is it requires more time to set up and align properly.

Accuracy :

When talking about rack & pinion accuracy, it can be broken down into three components:  Backlash, Pitch Deviation and Tooth Quality .

• Backlash is the amount of clearance between the rack & pinion tooth flanks, and will depend on the type of rack selected and alignment accuracy between the rack and pinion.  The backlash can be eliminated completely by using a preloaded split-pinion or dual pinion drive system .
   

• Pitch Deviation is the difference between the theoretical rack length and its actual length.  This varies depending on the rack quality selected and is shown in the catalog as GTf per 300 mm length.  Some servo systems can compensate for this error by using a laser interferometer to map out the errors.  Zero cumulative pitch deviation for long travels is available - please consult the factory.
   

• Tooth Quality is the accuracy that the tooth flanks are manufactured to, which affects the running precision and smoothness of the axis drive.  Soft rack & pinions have a quality level of ~AGMA 9 (DIN 9) *.  Induction-hardened rack & pinions have a quality level of ~AGMA 8 (DIN 10) *, due to deformation from the heat treatment process.  Quenched & Tempered rack & pinions have a quality level of ~AGMA 10 (DIN 8) *.  Finally, Hardened & Ground rack & pinions have a quality level of ~AGMA 12 (DIN 6) *, since the teeth are precision ground.  For high accuracy applications, a precision rack, such as the Hardened & Ground, should be used.

Lubrication :

With open gearing such as rack & pinions, lubrication is critical!  A thin film of grease or oil should always be on the contacting tooth flanks to ensure there is no metal-to-metal contact, which can damage the teeth.

It is recommended to use our Automatic Lubrication System, which pumps grease through a felt gear applicator and meshes directly to the pinion or the rack.  This provides continuous lubrication at the tooth flanks, extending the lifetime and maintaining the quality of the gearing.

For more information on our Automatic Lubrication System, please follow this link: Automatic Lubrication Systems.