Design Tools Technical Resources Technical Information: Screw Gears

Screw Gears: Technical Information

Characteristics∧ top

KHK stock screw gears come in modules 1 ~ 4, 10 to 30 teeth, and four materials: S45C, SUS303, CAC702 (formerly A BC2) and MC nylon.

Catalog No.	Module	Material	Heat Treatment	Tooth Surface Finish	Precision JIS B 1702-1 : 1998	Secondary Operations	Features
SN	1 ~ 4	S45C	—	Cut	N9	Possible	Popular screw gears. Additionally, gear tooth induction hardening secondary operations can be performed.
SUN	1 ~ 3	SUS303	—	Cut	N9	Possible	Suitable for food machinery due to SUS303's rust-resistant qualities.
AN	1 ~ 4	CAC702 (Al BC2)	—	Cut	N9	Possible	made of aluminum bronze, these gears have excellent wear resistance.
PN	1.5 ~ 3	MC901	—	Cut	N9	Possible	Light-weight products made of MC Nylon and can be used without lubrication.

Selection Hints∧ top

Please select the most suitable products by carefully considering the characteristics of items and contents of the product tables. It is also important to read all applicable “CAUTION” notes before the final selection. Since screw gears come in right- or left-hand helix, make sure to include the letter "R" or "L" in the catalog number when you order.

1. Caution in Selecting the Mating Gears
Screw gears are used for offset shafts. Whether the shafts are paralleled offset or skewed offset depends on the helix hands of the mating gears.

Direction of Shaft	Arrangement of Helix Hands
Skewed Shafts	RH-RH or LH-LH
Parallel Shafts	RH-LH

Arrangements of helix hands of screw gears

Trusted Quality Achieved by Years of Experience.
Efficient production by lapping cutting processes

Gear cutting by a hobbing machine

Application Hints∧ top

Rotation of two rollers by one input shaft

*The illustration above is a design example, not a design for machinery or a device in actual use.

2. Caution in Selecting Gears Based on Gear Strength
The allowable surface strengths listed in the product pages were derived using the Niemann formula as reference values (for the case of skewed offset shafts).

There is scarce amount of data on the strength of screw gears. The values of constant K₀ used in the calculations, which depend on the material of the mating gears, are our estimates. The mathematical expression below shows the Niemann formula to determine allowable tangential force F _t (kgf) and allowable torque T (kgf • m) on a basic circle.

The Niemann Formula

Value of f_z

Z₂	Z₁
Z₂	10	13	15	20	26	30
10	1.538
13	2.005	1.538
15	2.279	1.786	1.538
20	2.963	2.329	2.053	1.538
26	3.695	2.963	2.588	2.005	1.538
30	4.161	3.350	2.963	2.279	1.786	1.538

K₀ values depending on material combination

Catalog Number	Mating Gear	K₀	The maximum allowable sliding speed m/s	No. of teeth of mating gears	Rotation
SN	SN	0.0030	2.5	Same Number of Teeth	100 rpm
SUN	SN	0.0030 (Note 1)	2.5 (Note 1)
AN	SN	0.0050	5
PN	SN	0.0030 (Note 1) (0.0021)	2.5 (Note 1) (1.0)

(Note 1) K₀ values and the maximum allowable sliding speed of SUN and PN products are set by KHK. Screw gears are typically used with lubrication. In case of using PN products without lubrication, the parenthetical values shown in the table are applied.

Application Hints:
In order to use KHK stock screw gears safely, read the Application Hints carefully before proceeding. Also, please refer to the "Application Hints" in the technical information section on KHK stock spur gears, when performing secondary operations.

1. Points of Caution in Assembling
KHK stock screw gears are designed to give the proper backlash when assembled using the center distance given by the formula below with a tolerance of H7 to H8. The amount of backlash is given in the product table for each gear.

Backlash Screw Gears

(Caution) PN Plastic Screw Gears are excluded.

Due to their helix, screw gears produce axial thrust forces. The bearings must be selected properly to be able to handle these thrust forces. The directions of thrust change with the hand of helix and the direction os rotation as illustrated below.

Direction of Rotation and Thrust Force