Characteristics

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.

  • KWGDL
    Type : Duplex worm
    Module : 2-4
    Number of threads or reduction ratio : Single thread
    Material () JIS : SCM440
    Heat treatment : Thermal refined, gear teeth induction hardened
    Tooth surface finish : Ground
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 1
    Features : High-precision duplex worms with superior strength. A range of backlash values can be obtained by moving the worm axially.
  • KWGDLS
    Type : Duplex worm
    Module : 1.5-4
    Number of threads or reduction ratio : Single thread
    Material () JIS : SCM440
    Heat treatment : Thermal refined, gear teeth induction hardened
    Tooth surface finish : Ground
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 1
    Features : Duplex worms with a shaft, excellent in accuracy and strength. A range of backlash values can be obtained by moving the worm axially.
  • AGDL
    Type : Duplex worm gear
    Module : 1.5-4
    Number of threads or reduction ratio : 20-60
    Material () JIS : CAC702 (AlBC2)
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 1
    Features : Duplex worm wheels made of aluminum bronze, excellent in wear-resistance. The pitch accuracy is first grade.
  • KWG
    Type : Worm
    Module : 0.5-6
    Number of threads or reduction ratio : Single thread – Double thread
    Material () JIS : SCM440
    Heat treatment : Thermal refined, gear teeth induction hardened
    Tooth surface finish : Ground
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 2
    Features : Grounded finished worms with a shaft, including tooth surface quenching treatment. Allows compact design due to having small reference diameters.
  • AG *NOTE 1
    Type : Worm Gear
    Module : 0.5-1.5
    Number of threads or reduction ratio : 10-60
    Material () JIS : CAC702 (AlBC2)
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 2
    Features : Made of aluminum bronze, have excellent wear-resistance. Wide selection is available for this item.
  • AGF * NOTE 1
    Type :  Worm Gear
    Module : 2-6
    Number of threads or reduction ratio : 10-60
    Material () JIS : CAC702 (AlBC2)
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 2
    Features : Made of aluminum bronze, have excellent wear-resistance. Allows compact design.
  • SWG
    Type : Worm
    Module : 1-6
    Number of threads or reduction ratio : Single thread – Triple thread
    Material () JIS : S45C
    Heat treatment : Gear teeth induction hardened
    Tooth surface finish : Ground
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 2
    Features : Reasonably priced ground worms. Ready-to-use finished products from the J Series, are also available.
  • AG NOTE 1
    Type : Worm Gear
    Module : 1-6
    Number of threads or reduction ratio : 10-60
    Material () JIS : CAC702 (AlBC2)
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 2
    Features : Made of aluminum bronze, have excellent wear-resistance. Wide selection is available for this item.
  • SW
    Type : Worm
    Module : 0.5-6
    Number of threads or reduction ratio : Single thread – Double thread
    Material () JIS : S45C
    Heat treatment : –
    Tooth surface finish : Cut (Thread rolled)
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 4
    Features : Economical, commonly used worms that have broad utility. Ready-to-use finished products from the J Series are also available.
  • SUW
    Type : Worm
    Module : 0.5-3
    Number of threads or reduction ratio : Single thread – Double thread
    Material () JIS : SUS303
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 4
    Features : Rust-resistant worms made of stainless steel suitable for mating with DS or PG worm wheels. Finished products for the J Series are also available.
  • BG
    Type : Worm Gear
    Module : 0.5-6
    Number of threads or reduction ratio : 10-60
    Material () JIS : CAC502 (PBC2)
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 4
    Features : Phosphorous bronze worm wheels have excellent wear resistance. Interchangeable with CG Worm Wheels, and enhances strength.
  • CG
    Type : Worm Gear
    Module : 1-6
    Number of threads or reduction ratio : 10-120
    Material () JIS : FC200
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 4
    Features : Economical, commonly used worm wheels that have broad utility. Available with a large selection of modules and number of teeth.
  • DG
    Type : Worm Gear
    Module : 0.5-0.8
    Number of threads or reduction ratio : 10-60
    Material () JIS : Polyacetal
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 5
    Features : Fine pitch worm wheels made of polyacetal, a stable plastic material.
  • PG
    Type : Worm Gear
    Module : 1-3
    Number of threads or reduction ratio : 10-50
    Material () JIS : MC901
    Heat treatment : –
    Tooth surface finish : Cut
    Precision KHK W 001 / KHK W 002 *NOTE 2 : 5
    Features : 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 AGF and AG worm wheels is FC200(Cast Iron). AG worm wheels mate primarily with SWG worms. But, for Modules 0.8 or smaller, AG worm wheels mate with KWG 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.

worm_grinding
Worm Grinding Machine by Klingelnberg

worm_mating_test
Worm gear testing machine by Klingelnberg

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 calculations, please refer to the technical information contained in the “Surface Durability of Cylindrical Worm Gearing” section on Page 96.

Efficiency of KWGDLS / AGDL Worm Gear Pair (%)
(rpm = Rotation of worm)
Efficiency of KWGDLS AGDL Worm Gear Pair

Efficiency of KWG / AG, AGF Worm Gear Pair (%)
(rpm = Rotation of worm)
Efficiency of KWG, AG, AGF Worm Gear Pair

Efficiency of SW, SUM / CG, BG, PG Worm Gear Pair (%)
The efficiency is approximately as follows, depending on the assembly, loading, lubrication and rotational speed.

Catalog Number Thread Efficiency (%)
SU / SUW Single thread 40 – 50 %
Double thread 50 – 60 %

 

Efficiency of SWG / AG Worm Gear Pair (%)
(rpm = Rotation of worm)
Efficiency of SWG,AG Worm Gear Pair

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 degrees. For systems requiring fail-safe prevention of back drive, we recommend other braking mechanisms or one-way clutches.

Selection Hints

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.

1. Caution in Selecting the Mating Gears

Worms and worm wheels have either right-hand or left-hand helix. The same hand worms and worm wheels comprise sets. However, the number of threads and whether they use normal module or axial module system must also be matched. The table below shows available combinations of KHK stock worms and worm wheels.

Mating Worm Wheels Selection Chart
worm and worm gear matching chart
[NOTE 1] Select the same module for both members.

The Helixes of Worms and Worm Wheels
helix of worms and worm gear

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.

Calculation assumptions for Surface Durability

Catalog number – KWGDL & KWGDLS /AGDL, KWG/AGF, SWG/AG, SW/BG, SW/CG

  • Formula NOTE 2 / Formula of worm gear’s strength (JGMA405-01)
  • Rotations of worm / 600rpm (KWGDL & KWGDLS /AGDL, KWG/AGF, SWG/AG), 100rpm (SW/BG, SW/CG)
  • Lubricant / Lubricant for gears with proper viscosity and with anti-pressure additives
  • 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 (KWGDL & KWGDLS /AGDL, KWG/AGF, SWG/AG), 0.70 (SW/BG), 0.42 (SW/CG)

 

Calculation assumptions for Bending Strength

Catalog number – SUW/PG , SUW/DG

  • Formula NOTE 2 / The Lewis formula
  • Allowable bending stress (kgf/mm2) / 1.15 (40 deg C with No Lubrication) (SUW/PG), NOTE 3 1  (40 deg C with No Lubrication)

[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 DG worm wheel is the value we estimated.

The Maximum Allowable Sliding Speed Due to Heat

The maximum allowable sliding speed for each series of worm wheels is given on the right. Select the appropriate part by calculating the sliding speed.

Sliding speed vs (m/s)

vs = dn / 19100 cosγ

d :Worm pitch dia.
n :Worm speed (rpm)
γ :Worm nominal lead angle

Maximum Sliding Speed

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)
    Precision Grades of Worms
  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.
  3. Overall Length Tolerance of Worms
    Overall Length Tolerance of Worms
    Overall Length Tolerance of Worms
    Overall Length Tolerance of Worms Wheels
    Overall Length Tolerance of Worms Wheels
    [CAUTION] PG Plastic Wheels are excluded.
    Precision Grades of Worm Wheels (KHK W 002)
    Precision Grades of Worm Wheels (KHK W 002)
    Unit:μm

Application Hints

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 or your nearest distributor.

KHK Co., Ltd.
PHONE: 81-48-254-1744 FAX: 81-48-254-1765
E-mail info@khkgears.net

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 operation
    Fig.1
    If chucking operation using scroll chucks is to be done, we recommend the use of new or rebored jaws for improved precision.
  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 your KHK distributor.

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.
    Datum Clamp Face
  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 separate technical reference book for more details (Page 107).
    Direction of rotation and thrust force
    Direction of rotation and thrust force
  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 separate technical reference book for more details (Page 67).

Verify that the worm axis is perpendicular to the worm wheel axis.
Verify that the worm axis is perpendicular to the worm

Check that the worm axis is in the center of the worm wheel face width.
Check that the worm axis is in the center of the worm

Check the mounting distance (allowable mounting distance H7 – H8).
Check the mounting distance

Confirm that the center of the worm wheel goes through the midpoint of the worm length.
Confirm that the center of the worm wheel goes

Application Examples

SW Worms and CG Worm Wheels used in a rotating comb device
SW Worms and CG Worm Wheels used in a rotating comb device

SW Worms and BG Worm Wheels used in adjusting a cloth feeding device
SW Worms and BG Worm Wheels used in adjusting a cloth feeding device