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Worm Gears: Technical Information

Characteristics∧ top

The simplest way to obtain a large speed reduction with high torque in a compact space is with worm gear drives. KHK stock worms and worm wheels are available in modules 0.5 to 6 and in speed ratios of 1/10 to 1/120, made in a variety of materials and styles. We also offer stock duplex worms and worm wheels with which you can obtain a very low backlash, high rotational precision system. The following table lists the main features for easy selection.

Type Catalog No. Module No. of threads or reduction ratio Material ( ) JIS Heat treatment Tooth surface finish Precision KKHK W 001 KKHK W 002 Note 2 Features
Duplex Worms & Worm Wheels Worm KKWGDL 2~4 Single thread SCM440 Thermal refined, gear teeth induction hardened Ground 1 High-precision duplex worms with superior strength. A range of backlash values can be obtained by moving the worm axially.
Worm KKWGDLS 1.5~4 Single thread SCM440 Thermal refined, gear teeth induction hardened Ground 1 Duplex worms with a shaft, excellent in accuracy and strength. A range of backlash values can be obtained by moving the worm axially.
Worm Wheel KAGDL 1.5~4 20~60 CAC702 (AℓBC2) Cut 1 Duplex worm wheels made of aluminum bronze, excellent in wear-resistance. The pitch accuracy is first grade.
Worms & Worm Wheels Worm KKWG 0.5~6 Single thread - Double thread SCM440 Thermal refined, gear teeth induction hardened Ground 2 Grounded finished worms with a shaft, including tooth surface quenching treatment. Allows compact design due to having small reference diameters.
Worm Wheel KAG Note 1 0.5~1.5 10~60 CAC702 (AℓBC2) Cut 2 Made of aluminum bronze, have excellent wear-resistance. Wide selection is available for this item.
Worm Wheel KAGF Note 1 2~6 10~60 CAC702 (AℓBC2) Cut 2 Made of aluminum bronze, have excellent wear-resistance. Allows compact design.
Worm KSWG 1~6 Single thread - Triple thread S45C Gear teeth induction hardened Ground 2 Reasonably priced ground worms. Ready-to-use finished products from the J Series, are also available.
Worm Wheel KAG Note 1 1~6 10~60 CAC702 (AℓBC2) Cut 2 Made of aluminum bronze, have excellent wear-resistance. Wide selection is available for this item.
Worm KSW 0.5~6 Single thread - Double thread S45C Cut (Thread rolled) 4 Economical, commonly used worms that have broad utility. Ready-to-use finished products from the J Series are also available.
Worm KSUW 0.5~3 Single thread - Double thread SUS303 Cut 4 Rust-resistant worms made of stainless steel suitable for mating with KDS or KPG worm wheels. Finished products for the J Series are also available.
Worm Wheel KBG 0.5~6 10~60 CAC502 (PBC2) Cut 4 Phosphorous bronze worm wheels have excellent wear resistance. Interchangeable with CG Worm Wheels, and enhances strength.
Worm Wheel KCG 1~6 10~120 FC200 Cut 4 Economical, commonly used worm wheels that have broad utility. Available with a large selection of modules and number of teeth.
Worm Wheel KDG 0.5~0.8 10~60 Polyacetal Cut 5 Fine pitch worm wheels made of polyacetal, a stable plastic material.
Worm Wheel KPG 1~3 10~50 MC901 Cut 5 Light weight and strong MC Nylon worm wheels. Suitable for use in food machinery, and can be used without lubricant.
NOTE 1: The material of cast hubs for KAGF and KAG worm wheels is FC200(Cast Iron). KAG worm wheels mate primarily with KSWG worms. But, for Modules 0.8 or smaller, KAG worm wheels mate with KKWG worms. NOTE 2: KHK stock worms and worm wheels are produced to KHK's own precision grades. See the "Precision of Worms and Worm Wheels" in the "Selection Hints" section.
Our precision gear cutting technology enables acceleration and noise reduction
Setting the proper tooth contact and the backlash is essential for using worm gears. Use KHK stock worm gears for safe, reliable use.

Grinding Machine Testing Machine


1. Efficiency of Worm Gear Pair The efficiency of power transmission varies somewhat with the conditions of assembly and lubricant, but is generally 30~90% (excludes losses from bearings and churning of lubricants). The efficiency of KHK stock worm gear pair is given below as a reference. To learn more about strength calculation, please refer to the technical information contained in the "Surface Durability of Cylindrical Worm Gearing".

Efficiency of KWGDLS/AGDL Worm Gear Pair (%) (rpm = Rotation of worm)
Worm rpm Catalog No. 100 300 600 900 1200 1800
KKWGDL1.5-R1 35 42 47 51 53 57
KKWGDL2-R1 38 45 51 55 56 61
KKWGDL2.5-R1 40 48 54 57 60 63
KKWGDL3-R1 41 49 55 58 62 65
KKWGDL3.5-R1 42 50 56 61 62 65
KKWGDL4-R1 42 51 56 61 63 67

Efficiency of KWG/AG, AGF Worm Gear Pair (%) (rpm = Rotation of worm)
Worm rpm Catalog No. 100 300 600 900 1200 1800
KKWG0.5-R1 30 34 38 41 43 46
KKWG0.8-R1 35 40 44 47 49 53
KKWG1-R1 34 40 45 48 51 54
KKWG1.5-R1 35 42 47 51 53 57
KKWG2-R1 45 51 56 60 62 65
KKWG2.5-R1 44 51 57 61 62 67
KKWG3-R1 44 52 58 61 64 67
KKWG4-R1 50 58 64 66 70 72
KKWG5-R1 51 60 66 69 71 73
KKWG6-R1 53 61 66 70 72 75
KKWG0.5-R2 46 50 54 58 60 63
KKWG0.8-R2 51 56 61 64 66 69
KKWG1-R2 51 56 62 64 67 70
KKWG1.5-R2 52 59 64 67 69 73
KKWG2-R2 61 67 71 74 76 78
KKWG2.5-R2 60 67 72 75 76 80
KKWG3-R2 61 68 73 75 78 80
KKWG4-R2 66 73 77 79 82 84

Efficiency of KSW, KSUM / KCG, KBG, KPG Worm Gear Pair (%)
The efficiency is approximately as follows, depending on the assembly, loading, lubrication and rotational speed.
Catalog No. Thread Efficiency (%)
KSW/KSUW Single Thread 40 ~ 50%
Double Thread 50 ~ 60%

Efficiency of SWG/AG Worm Gear Pair (%) (rpm = Rotation of worm)
Worm rpm Catalog No. 100 300 600 900 1200 1800
KSWG1-R1 34 40 45 48 51 54
KSWG1.5-R1 35 42 47 51 53 57
KSWG2-R1 38 45 51 55 56 61
KSWG2.5-R1 40 48 54 57 60 63
KSWG3-R1 41 49 55 58 62 65
KSWG4-R1 42 51 56 61 63 67
KSWG5-R1 46 54 60 64 66 70
KSWG6-R1 48 57 64 66 68 73
KSWG1-R2 51 56 62 64 67 70
KSWG1.5-R2 52 59 64 67 69 73
KSWG2-R2 55 62 67 70 72 75
KSWG2.5-R2 57 64 69 72 75 77
KSWG3-R2 58 66 71 73 76 78
KSWG4-R2 59 67 72 75 77 80
KSWG5-R2 62 70 75 78 79 82
KSWG6-R2 65 72 77 80 81 84
KSWG3-R3 67 74 78 80 82 84
KSWG4-R3 68 75 79 82 83 86

2. Self-Locking Feature of Worm Gear Pair
Self-locking is defined as the inability of worm wheels to drive the worms. Factors affecting the self-locking feature include the materials of the worm and worm wheel, lead angle, precision of manufacture, types of bearings, lubricant, etc. Thus, it is not dependent simply on the lead angle. But, in general, self-locking will occur when the lead angle in a single thread worm is less than 4°. For systems requiring fail-safe prevention of back drive, we recommend other braking mechanisms or one-way clutches.

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 shown below before the final selection. Use of catalog numbers when ordering will simplify and expedite the processing of your order.

Mating Worm Wheels Selection Chart
Worm KKWGDL KKWGDLS KKWG KSWG KSW KSUW
Mating Worm Wheel NOTE 1 Helix/Thread R1 R1 R2 R1 R2 R3 R1 R2 L1 L2 R1 R2
KAGDL R1 O
KAG0.5~1.5 KAGF R1 O
R2 O
KAG R1 O
R2 O
R3 O
KBG R1 O O
R2 O O
L1 O
L2 O
KCG R1 O O
R2 O O
L1 O
L2 O
KPG R1 O O
R2 O O
KDG R1 O O
R2 O O
NOTE 1: Select the same module for both members. Helixes

2. Caution in Selecting Gears Based on Gear Strength
The gear strength values shown in the product pages were compute by assuming a certain application environment as shown below. Therefore, they should be used as reference only. We recommend that each user computes their own values by applying the actual usage conditions.

Catalog No. / Item KKWGDL, KKWGDLS/AGDL, KKWG/AGF, KSWG/AG KSW/KBG KSW/KCG KSUW/KPG KSUW/KDG
Calculation Assumptions for Surface Durability
Formula NOTE 2 Formula of worm gears strength (JGMA405-01) The Lewis formula
Rotations of worm 600rpm 100rpm Allowable bending stress(kgf/mm²)
Lubricant Lubricant for gears with proper viscosity and with anti-pressure additives
1.15
(40°C with No Lubrication)
Note 1 1
(40°C with No Lubrication)
Lubrication Oil Bath
Starting condition Starting torque less than 200% of rated torque. Less than 2 starts per hour
Durability 26000 hours
Impact from motor Uniform load
Impact from Load Uniform load
Allowable stress factor Sclim 0.67 0.70 0.42
NOTE 2: The gear strength formula is based on JGMA (Japanese Gear Manufacturer's Association) specifications and "MC Nylon Technical Data" by Nippon Polypenco Limited. The units for the rotational speed (rpm) and the stress (kgf/mm2) are adjusted to the units needed in the formula. NOTE 3: Allowable bending stress of KDG worm wheel is the value we estimated.
Catalog No. Max. Sliding Speed (m/s)
KGDL * 15
KAGL * 15
KAG * 15
KBG * 10
KCG * 2.5
KPG 1 (no lubrication)
* JGMA405-01
The Maximum Allowable Sliding Speed Due to Heat
The maximum allowable sliding speed for each series of worm wheels is given pn the right. Select the appropriate part by calculating the sliding speed.

Sliding speed vs (m/s)
Formula
3. Selecting Worms and Worm Wheels by Precision
The precision standards of KHK stock worms and worm wheels are established by us. The table below indicates the tolerance ranges for our products.
  1. Precision of worms (KHK W 001)
    KHK established allowable profile and lead errors of worms with precision grades 1 to 4, by using the JIS Standard as reference. Lead errors are measured over one full revolution.

    Precision Grades of Worms (KHK W 001) (Unit: μm)
    Grade Error Module
    over m0.4 up to1 over m1up to 1.6 over m1.6 up to 2.5 over m2.5 up to 4 over m4 up to 6
    1 Tooth profile error 8 12 16 20 25
    Lead error 7 9 11 13 16
    2 Tooth profile error 12 16 20 24 29
    Lead error 15 18 21 25 28
    3 Tooth profile error 16 23 30 37 50
    Lead error 20 23 27 33 37
    4 Tooth profile error 20 30 40 50 70
    Lead error 30 32 38 46 52

  2. Precision of worm wheels (KHK W 002)
    We have established standard grades 1 to 5 of worm wheels using the JIS Standard as reference. The allowable values of Single Pitch Error and Runout Error are defined for each module size and pitch diameter.

    Precision Grades of Worm Wheels (KHK W 002)
    Grade Error Over m0.4 up to 1 Over m1 up to 1.6 Over m1.6 up to 2.5 Over m2.5 up to 4 Over m4 up to 6
    Pitch diameter (mm)
    6 up to 12 12 up to 25 25 up to 50 50 up to 100 100 up to 200 12 up to 25 25 up to 50 50 up to 100 100 up to 200 200 up to 400 12 up to 25 25 up to 50 50 up to 100 100 up to 200 200 up to 400 25 up to 50 50 up to 100 100 up to 200 200 up to 400 400 up to 800 25 up to 50 50 up to 100 100 up to 200 200 up to 400 400 up to 800
    1 Single pitch error 5 6 7 7 9 6 7 8 9 10 7 7 8 9 11 8 9 10 11 13 9 10 11 13 14
    Total composite error 21 24 26 30 34 25 28 31 35 41 27 30 33 37 43 33 36 40 46 53 37 40 45 50 57
    2 Single pitch error 8 8 9 10 12 9 10 11 12 14 9 10 12 13 15 11 13 14 16 18 13 14 16 18 20
    Total composite error 30 33 37 42 48 35 39 44 50 57 38 42 46 52 60 46 51 57 64 74 52 57 63 71 80
    3 Single pitch error 11 12 13 15 17 12 14 16 18 20 13 15 16 19 21 16 18 20 23 26 19 20 22 25 29
    Total composite error 43 47 53 60 68 50 55 62 71 81 53 59 66 74 85 65 72 81 91 105 74 81 90 100 115
    4 Single pitch error 15 17 19 21 24 18 19 22 25 29 19 21 23 26 30 23 25 28 32 37 26 28 32 35 40
    Total composite error 60 66 74 83 95 70 77 87 99 115 75 83 92 105 120 91 100 115 130 145 105 115 125 140 160
    5 Single pitch error 21 24 26 30 34 25 28 31 35 41 27 30 33 37 43 33 36 40 46 53 37 40 45 50 57
    Total composite error 86 94 105 120 135 100 110 125 140 165 105 120 130 150 170 130 145 160 185 210 150 160 180 200 230

  3. Overall Length Tolerance of Worms
    Series Total length (mm) Tolerance
    KWGDL Uniform 0
    -0.10
    SWG, KSW, KSUW Less than 100 0
    -0.15
    Over 100 0
    -0.20
    KKWGDLS, KKWG Uniform Normal tolerance

    Overall Length Tolerance of Worms Wheels
    Total length (mm) Tolerance
    Below 30 0
    -0.10
    over 30 up to 100 0
    -0.15
    Over 100 0
    -0.20
    CAUTION KPG Plastic Wheels are excluded.

Application Hints∧ top

In order to use KHK stock worms and worm wheels safely, carefully read the Application Hints before proceeding. If there are questions or you require clarifications, please contact our technical department.

1. Caution on Performing Secondary Operations
  1. If you are reboring, it is important to pay special attention to locating the center in order to avoid runout. (Fig.1) The reference datum for gear cutting or grinding is the bore. (For worm shafts, it is ground portion of the shaft.) Therefore, use the bore or shaft for locating the center. If it is too difficult to do for small bores, the alternative is to use one spot on the bore and the runout of the side surface.
    Lathe Operations
  2. To open up the bore to its maximum, calculate the bore size so that the tooth strength is weaker than the strength of the remaining material. For machining the maximum bore diameter, it should be designed so that the thickness between hub diameter (or root diameter) to bore diameter has more strength than the gear strength. As a guide, the maximum machined bore diameter should be within 60% to 70% of the hub diameter (or root diameter). When the keyway is processed, it should be 50% to 60%. In the case FC material is used, it should be lower by 10% or more.
  3. Since worm wheels are molded products, they may have air bubbles inside the material. In case you find air bubbles inside when performing secondary operations, and if the bubbles are found to be troublesome, please contact us.

2. Points of Caution in Assembling
  1. KHK stock worms and worm wheels are designed such that when assembled according to the specified Mounting distance with a tolerance of H7 to H8, the backlash shown in the product tables is obtained. Do not attempt to eliminate backlash by pushing worms into worm wheels or operate with the worm shifted in the direction along the tooth.
  2. The figure below shows the datum clamp face of a worm wheel. When assembling worm gears, be sure that the worm axis is in the center of the worm wheel face width.
    Thrust Force
  3. Because of the helix of the gear teeth, worms and worm wheels produce axial thrust forces. The directions of thrust depend on the hand of the helix and the direction of rotation. This is illustrated below in Fig.2. The bearings must be selected properly to be able to handle these thrust forces. See the "Gear Forces" section in the technical reference.
  4. Because large thrust forces act on worms, if they are not secured to the shaft firmly, they tend to shift. Use of step shafts, set screws, dowel pins, etc., are recommended. Also, check for loosening of bearings due to thrust forces.

3. Verifying the Orientation of Assembly How well the worms and worm wheels are assembled has large effects on the friction of the unit. The tooth contact at the time of assembly must be checked for correctness as shown below. See the "Tooth Contact of a Worm Gear Pair" section in the technical reference>

• Verify that the worm axis is perpendicular to the worm wheel axis.
Verify 1
• Check that the worm axis is in the center of the worm wheel face width.
Verify 2
• Check the Mounting distance (allowable Mounting distance H7~H8).
Verify 3
• Confirm that the center of the worm wheel goes through the midpoint of the worm length.
Verify 4

Application Examples
Example 1
Example 2
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