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
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Efficient production by lapping cutting processes Gear cutting by a hobbing machine
*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 fz
Z2
Z1
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.
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.
(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 (Caution) For parallel shaft applications, see the "Application Hints" for KHK Helical Gears.
(646) 396-GEAR
Gear cutting by a hobbing machine
Application Hints: 
