Gear Racks

Helical	Straight
These racks have the teeth at an angle (19º 31' 42") to rack length for increased load capacity and quiet operation.	These racks have the teeth perpendicular to rack length, and are traditionally known as "spur".

Helical versus Straight:  The helical style provides several key benefits over the straight style, including:

• They run quieter than the straight, especially at high speeds
• They have a higher contact ratio (the number of effective teeth engaged) than straight, which increases the load carrying capacity
• Their 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, they cost the same as the straight racks!

There are four classes of gear racks available and eight different quality levels, each of which is well suited for certain types of applications.  Click on Series Numbers for PDF catalog pages.

Rack Class:	Rack Type:	Tooth Quality:	Pitch Accuracy:	Typical Applications:	Helical:	Straight:
Basic (BR)	Induction Hardened	DIN 10 (~AGMA 8)	< 0.200 mm per meter	Heavy Loads Low Accuracy	39-Series	27-Series 34-Series
	Soft	DIN 9 (~AGMA 9)	< 0.150 mm per meter	Light Loads Medium Accuracy	47-Series	25-Series 36-Series (Stainless)
Precision (PR)	Quenched & Tempered	DIN 8 (~AGMA 10)	< 0.100 mm per meter	Medium Loads Medium Accuracy	38-Series	33-Series
	Hardened & Ground	DIN 8 (~AGMA 10)	< 0.060 mm per meter	Heavy Loads High Accuracy	29-Series	28-Series
High Precision (HPR)	Hardened & Ground	DIN 7 (~AGMA 11)	< 0.052 mm per meter	Heavy Loads High Accuracy	29-Series	28-Series
		DIN 6 (~AGMA 12)	< 0.036 mm per meter		29-Series	28-Series
Ultra-High Precision (UHPR)	Hardened & Ground	DIN 5	< 0.026 mm per meter	Heavy Loads Ultra-High Accuracy	29-Series	28-Series
		DIN 3	< 0.012 mm per meter		48-Series	46-Series

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.

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.  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, smoothness, backlash and noise level of the axis drive.  Soft racks have a quality level of ~AGMA 9 (DIN 9).  Induction-hardened racks have a quality level of ~AGMA 8 (DIN 10), due to deformation from the heat treatment process.  Quenched & Tempered racks have a quality level of ~AGMA 10 (DIN 8).  Finally, Hardened & Ground racks have a quality level of ~AGMA 12 (DIN 6), since the teeth are precision ground.  For high accuracy applications, a Precision Hardened & Ground Rack should be used.

*AGMA quality level numbers increase with higher precision; DIN quality level numbers decrease with higher precision.

Selecting the right gear rack and pinion 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.

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

* Ratings are based on helical rack with a 25 tooth hardened & ground pinion; using smaller pinions would reduce rating.  The ratings are based on fatigue strength with adequate lubrication; breaking strength ratings are much higher.  Service factors need to be applied to these ratings to make a proper selection.  The rack mounting screw holding force may also need to be checked.

Helical Rack Ratings  |  Straight Rack Ratings

What is a Module?  Module is a metric pitch, inversely related to diametral pitch, e.g. Module = 25.4 / Diametral Pitch.

For application & selection assistance, please call us or fill out our Application Sheet.  Your application data will be reviewed by our engineers, who will recommend the best solution for your application.

Rack & Pinion Selection Procedure  |  Application Sheet

To view the rack dimensions, please click on the "Series" in left column.

Series	Style	Rack Description	Modules Available	Tooth Heat Treatment	Quality Level
28	Straight	Hardened & Ground	2, 3, 4, 5, 6 8, 10, 12	Case-Hardened	DIN 6 (~AGMA 12)
28a	Straight	Hardened & Ground	2, 3, 4, 5, 6, 8	Induction-Hardened	DIN 7 (~AGMA 11)
28b	Straight	Hardened & Ground	2, 3, 4, 5	Induction-Hardened	DIN 8 (~AGMA 10)
29	Helical	Hardened & Ground	2, 3, 4, 5, 6 8, 10, 12	Case-Hardened	DIN 6 (~AGMA 12)
29a	Helical	Hardened & Ground	2, 3, 4, 5, 6, 8, 10	Induction-Hardened	DIN 7 (~AGMA 11)
29b	Helical	Hardened & Ground	2, 3, 4, 5, 6	Induction-Hardened	DIN 8 (~AGMA 10)
33	Straight	Quenched & Tempered	2, 3, 4	Quenched & Tempered	DIN 8 (~AGMA 10)
34	Straight	Induction Hardened	2, 3, 4, 5, 6 8, 10, 12	Induction Hardened	DIN 10 (~AGMA 8)
38	Helical	Quenched & Tempered	2, 3, 4	Quenched & Tempered	DIN 8 (~AGMA 10)
39	Helical	Induction Hardened	2, 3, 4, 5, 6 8, 10, 12	Induction Hardened	DIN 10 (~AGMA 8)
47	Helical	Soft	1.5, 2, 3, 4 5, 6, 8, 10	None	DIN 9 (~AGMA 9)

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 Systems, please follow this link.