How To Use Worm Gears

Features

KHK stock worm gears are available in modules 0.5 to 6, reduction ratios of 1/10 to 1/120, and a wide range of materials and shapes. Duplex worm gears with adjustable backlash and highly accurate rotation are also available. The following table lists the main features.

Type		Catalog Number	Module	Number of Starts or Reduction Ratio	Material Old JIS in parentheses	Heat treatment	Tooth Surface Finish	Precision Grade KHK W 001 KHK W 002 Note 2	Features
Duplex Worm Gears	Worms	KWGDL	2 - 4	Single Start	SCM440	Thermal refined, gear teeth induction hardened	Ground	1	Duplex worms that have been tempered, hardened and ground that has excellent accuracy, strength and abrasion resistance. Secondary operations can be given except for the teeth. Moving it in axial direction will adjust the backlash.
	Worms	KWGDLS	1.5 - 4	Single Start	SCM440	Thermal refined, gear teeth induction hardened	Ground	1	Duplex worms with shafts that have been tempered, hardened and ground that has excellent accuracy, strength and abrasion resistance. Secondary operations can be given except for the teeth. Moving it in axial direction will adjust the backlash.
	Wheels	AGDL	1.5 - 4	20 - 60	CAC702 (AℓBC2)	-	Cut	1	Duplex worm wheels made of aluminum bronze with excellent accuracy and a good balance between machinability and wear resistance. Used in combination with KWGDL or KWGDLS.
Worm Gears	Worms	KWG	0.5 - 6	Single or Double Start	SCM440	Thermal refined, gear teeth induction hardened	Ground	2	Worms with shafts that have been tempered, hardened and ground that has excellent accuracy, strength and abrasion resistance. Secondary operations can be given except for the teeth.
	Wheels	AGF NOTE 1	2 - 6	10 - 60	CAC702 (AℓBC2)	-	Cut	2	Worm wheels made of aluminum bronze with a good balance between machinability and wear resistance. Used in combination with KWG.
	Worms	SWG	1 - 6	Single or Double or Triple Start	S45C	Gear teeth induction hardened	Ground	2	Worms that have been hardened and ground with a good balance of accuracy, wear resistance and cost. Secondary operations are possible except for the teeth.
	Wheels	AG NOTE 1	1 - 6	10 - 60	CAC702 (AℓBC2)	-	Cut	2	Worm wheels made of aluminum bronze with a good balance between machinability and wear resistance. Used in combination with SWG. Note 1
	Worms	SW	0.5 - 6	Single or Double Start	S45C	-	Cut (Rolling)	4	Many lineups are available at a low price and excellent usability.
	Worms	SUW	0.5 - 3	Single or Double Start	SUS303	-	Cut	4	Stainless steel worms with rust resistance.
	Wheels	BG	0.5 - 6	10 - 60	CAC502 (PBC2)	-	Cut	4	Worm wheels made of phosphorus bronze with excellent wear resistance. Used in combination with SW and SUW.
	Wheels	CG	1 - 6	10 - 120	FC200	-	Cut	4	Cast iron worm wheels that are inexpensive and suitable for light loads. Used in combination with SW and SUW.
	Wheels	DG	0.5-0.8	10 - 60	Polyacetal	-	Cut	5	Worm wheels made of polyacetal. Used in combination with SW and SUW.
	Wheels	PG	1 - 3	10 - 50	MC901	-	Cut	5	Worm wheels made of MC nylon. Can be used with no lubrication. Used in combination with SW and SUW.

[Note 1]
FC200 is the material for the hubs of AGF and AG worm wheels. The AG worm wheels are normally combined with the SWG worms, but are also compatible with the KWG worms with module 1.5 and below.
[Note 2]
The precision grade of KHK stock worm gears controls the product quality based on the KHK standards. Please see “Precision of Worm Gears” in Selection Hints section for details.

High-precision ground gear worms are available.
We use screw grinding machines manufactured by DRAKE, USA, to manufacture high-precision ground worms of module 0.5 to 8.

CNC Screw Grinding Machine (TE-LM200)

Worm ground gear machining range
Maximum gear accuracy	KHK Grade 1
Maximum module	m8
Maximum nominal lead angle	±35°
Maximum outer diameter	φ200mm
Maximum length	330mm

Application Examples

KHK stock worm gears are used in a wide range of fields, including reduction gears and positioning mechanisms.

Wiper Drive Device

Worm gear used for the oscillating mechanism of wipers

Yaesu Steam Kettle

Yaesu Steam Kettle
SW worm and CG worm wheel used for rotating large pans

Masdac Food Filling Device

Masdac Food Filling Device
KWGDL duplex worm and AGDL duplex wheel used for indexing and driving, for accurate filling of a fixed amount of ingredients

Gonio Stage Design Example

Gonio Stage Design Example
Worm gear used for rotating tables (design example)

Fabric Feeding Device

Fabric Feeding Device
SW worm and BG worm wheel used for adjusting height

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.

1. Caution in Selecting the Mating Gears

Worm gears are available in right-hand helix and left-hand helix. Worms and worm wheels of the same helix direction are combined. However, combination may not be possible due to the difference in the number of starts of the worm and the difference in pitch being normal and transverse (axial direction). Below is the Mating Helical Gear Selection Chart for KHK worm gears.

Mating Worm Wheel Selection Chart

[Note 1] The mating wheel must have the same module as the worm.

Helix direction of worm gears

2. Caution in Selecting Gears Based on Gear Strength

The gear strength values shown in the product pages were computed by assuming the application environment in the table below. Therefore, they should be used as reference only. We recommend that each user computes their own values by applying the actual usage conditions.

Item	Catalog Number
Calculation of Surface Durability				Calculation of Bending Strength of Gears
Item	KWGDL/KWGDLS/AGDL/KWG/AGF, SWG/AG	SW/BG	SW/CG	SUW/PG	SUW/DG
Formula NOTE 1	Formula of cylindrical worm gear strength (JGMA405-01)			The Lewis formula
Lubricating Oil	Lubricating oil with appropriate viscosity that contains extreme pressure additive for gears			Allowable bending stress (kgf/mm2)
Lubrication Method	Oil Bath Lubrication (Oil Bath)			1.15 (40°C with No Lubrication)	Note 2 1 (40°C with No Lubrication)
Startup Status	The starting torque is 200% or less of the rated torque, and the number of starts per hour is under 2.
Expected Service Life	26,000 hours
Impact from motor	Uniform load
Impact from load	Uniform load
Allowable Stress Coefficient Sclim	0.67	0.70	0.42

[NOTE 1]
The gear strength formula is based on JGMA (Japanese Gear Manufacturers Association) specifications, “MC Nylon Technical Data” by Mitsubishi Chemical Advanced Materials. The units for the rotational speed (rpm) and the stress (kgf/mm2) are adjusted to the units needed in the formula.
[NOTE 2]
The values for DG worm wheels were assumed by KHK.

When selecting KHK standard gears, glance over the Cautions on Product Characteristics and Cautions on Performing Secondary Operations in the respective dimension tables.

1.
Products not listed in this catalog or materials, modules, number of teeth and the like not listed in the dimensional tables can be manufactured as custom items. Please see Page 24 for more details about custom-made orders.
2.
The color and shape of the product images listed on the dimension table page of each product may differ from the actual product. Be sure to confirm the shape in the dimension table before selection.
3.
The details (specifications, dimensions, etc.) listed in the catalog may be changed without prior notice. Changes are announced on the KHK website.

Website URL : https://khkgears.net/new/
Overseas Sales Department : Phone: +81-48-254-1744 Fax: +81-48-254-1765 E-mail: info@khkgears.net

The most important factor in selecting gears is the gear strength.

Step 1

Use the calculation of load torque applied to the gear and the sliding speed to determine the worm gear suitable for the purpose.

Maximum allowable sliding speed due to friction

The maximum allowable sliding speed of each worm gear is shown in the table below. Sliding speed should be calculated when making a selection.

Sliding speed νs (m/s)

νs = dn 19100 cosγ

d : Worm pitch dia.
n : Worm rotational speed
γ : Worm nominal lead angle

Catalog Number	Maximum allowable sliding speed (m/s)
AGDL	* 15
AGF	* 15
AG	* 15
BG	* 10
CG	* 2.5
PG	1 (No Lubrication)

* From JGMA405-01

Surface durability

Calculated values of the strength at which the worm wheel teeth do not wear due to surface fatigue damage.
If the tooth surface is not sufficiently lubricated, the surface may be damaged even if the load is less than the tooth surface durability.

Step 2

Select provisionally from the allowable torque table of the Master Catalog or Web Catalog based on the load torque.

For provisional selection from the Master Catalog

Step 3

Calculate the strength under the actual usage conditions. Calculate the strength using the various gear strength formulas.
Please see our separate technical reference book for more details.

For strength calculation from the Master Catalog

For strength calculation from the Web Catalog

3. Cautions on Selecting Racks By Precision

The precision standards of KHK stock worm gears are established by us. Check the precision table below.

1. Worm Precision (KHK W 001)

For the pitch error and lead error of the worm, the allowable values of 1 to 4 grades are set for each module with reference to the JIS standards. The lead error is the allowable value of the tooth trace error in one lead.

Worm Precision KHK W 001 (Unit: μm)

Grade	Error	Module
Grade	Error	Over m0.4 to 1	Over m1 to 1.6	Over m1.6 to 2.5	Over m2.5 to 4	Over m4 to 6
1	Pitch Error	8	12	16	20	25
1	Lead Error	7	9	11	13	16
2	Pitch Error	12	16	20	24	29
2	Lead Error	15	18	21	25	28
3	Pitch Error	16	23	30	37	50
3	Lead Error	20	23	27	33	37
4	Pitch Error	20	30	40	50	70
4	Lead Error	30	32	38	46	52

2. Worm Wheel Accuracy (KHK W 002)

Our precision grades for pitch errors are established by referring to old JIS Standards. The precision grades are set from 1 to 5, in accordance with the tolerance of a single pitch error (S.P.E.), adjacent tooth-to-tooth error (T.T.E.), and the total composite error (T.C.E.) for each module and pitch diameter.

Worm Wheel Accuracy KHK W 002 (Unit: μm)

Grade	Error	Over m0.4 to 1					Over m1 to 1.6					Over m1.6 to 2.5					Over m2.5 to 4					Over m4 to 6
		Pitch dia. (mm)
		6 to 12	12 to 25	25 to 50	51 to 100	100 to 200	12 to 25	25 to 50	51 to 100	100 to 200	200 to 400	12 to 25	25 to 50	51 to 100	100 to 200	200 to 400	25 to 50	51 to 100	100 to 200	200 to 400	400 to 800	25 to 50	51 to 100	100 to 200	200 to 400	400 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
1	Total 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
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
2	Total Pitch 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
3	Total Pitch 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
4	Total Pitch 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
5	Total Pitch 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. Total Length Tolerance for Worm Gears

Total Length Tolerance for Worms

Series	Total Length (mm)	Tolerance
KWGDL	Uniform	0 -0.10
SWG SW SUW	100 or less	0 -0.15
SWG SW SUW	Over 100 200 or less	0 -0.20
KWGDLS KWG	Uniform	Normal Tolerance

Total Length Tolerance for Worm Wheels

Total Length (mm)	Tolerance
30 or less	0 -0.10
31 to 100	0 -0.15
Over 100	0 -0.20

[NOTE] PG Plastic Wheels are excluded.

4. Cautions in Selecting Worm Gears Based on Efficiency

The transmission efficiency of worm gears varies slightly depending on the assembled state, lubricating oil and the like, but the transmission efficiency (excludes bearing loss and loss of lubricating oil due to stirring) of worm wheels when driven from the worm is approximately 30% to 90%. The transmission efficiency table of the KHK stock worm gears is shown below (reference values). For details, refer to the “Formula of cylindrical worm gear strength” section in our separate technical reference book.

1. Efficiency of Various Worms

KWGDL/KWGDLS/AGDL Worm Gear Efficiency (%)

(rpm = Worm Rotational Speed)

Catalog Number	Worm Rotational Speed
Catalog Number	100	300	600	900	1200	1800
KWGDL1.5-R1	35	42	47	51	53	57
KWGDL2-R1	38	45	51	55	56	61
KWGDL2.5-R1	40	48	54	57	60	63
KWGDL3-R1	41	49	55	58	62	65
KWGDL3.5-R1	42	50	56	61	62	65
KWGDL4-R1	42	51	56	61	63	67

KWG/AG/AGF Worm Gear Efficiency (%)

(rpm = Worm Rotational Speed)

Catalog Number	Worm Rotational Speed
Catalog Number	100	300	600	900	1200	1800
KWG0.5-R1	30	34	38	41	43	46
KWG0.8-R1	35	40	44	47	49	53
KWG1-R1	34	40	45	48	51	54
KWG1.5-R1	35	42	47	51	53	57
KWG2-R1	45	51	56	60	62	65
KWG2.5-R1	44	51	57	61	62	67
KWG3-R1	44	52	58	61	64	67
KWG4-R1	50	58	64	66	70	72
KWG5-R1	51	60	66	69	71	73
KWG6-R1	53	61	66	70	72	75
KWG0.5-R2	46	50	54	58	60	63
KWG0.8-R2	51	56	61	64	66	69
KWG1-R2	51	56	62	64	67	70
KWG1.5-R2	52	69	64	67	69	73
KWG2-R2	61	67	71	74	76	78
KWG2.5-R2	60	67	72	75	76	80
KWG3-R2	61	68	73	75	78	80
KWG4-R2	66	73	77	79	82	84

SWG/AG Worm Gear Efficiency (%)

(rpm = Worm Rotational Speed)

Catalog Number	Worm Rotational Speed
Catalog Number	100	300	600	900	1200	1800
SWG1-R1	34	40	45	48	51	54
SWG1.5-R1	35	42	47	51	53	57
SWG2-R1	38	45	51	55	56	61
SWG2.5-R1	40	48	54	57	60	63
SWG3-R1	41	49	55	58	62	65
SWG4-R1	42	51	56	61	63	67
SWG5-R1	46	54	60	64	66	70
SWG6-R1	48	57	64	66	68	73
SWG1-R2	51	56	62	64	67	70
SWG1.5-R2	52	59	64	67	69	73
SWG2-R2	55	62	67	70	72	75
SWG2.5-R2	57	64	69	72	75	77
SWG3-R2	58	66	71	73	76	78
SWG4-R2	59	67	72	75	77	80
SWG5-R2	62	70	75	78	79	82
SWG6-R2	65	72	77	80	81	84
SWG3-R3	67	74	78	80	82	84
SWG4-R3	68	75	79	82	83	86

SW, SUW/CG, BG, PG Worm Gear Efficiency

The values shown in the table below are estimates and may vary slightly according to conditions such as assembled state, load, lubrication and rotation speed.

Catalog Number	Number of Starts	Efficiency (%)
SW/SUW	1 Start	40 - 50%
SW/SUW	2 Starts	50 - 60%

2. Self-locking of worm gears

The state of the worm not being able to rotated from the worm wheel is called self-locking. Self-locking happens due to the worm gear’s material, lead angle, machining accuracy, bearing type, lubricating oil, etc.
There are various factors, so self-lock is not always determined only by the lead angle, but usually a single-row worm self-locks at a lead angle of 4° or less. If complete reverse prevention is required, use another braking mechanism or the like in combination.

Application Hints

In order to use KHK stock worm gears 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.
E-mail : info@khkgears.net

1. Cautions on Handling

1.
KHK products are packaged one by one to prevent scratches and dents, but if you find issues such as rust, scratches, or dents when the product is removed from the box after purchase, please contact the supplier.
2.
Depending on the handling method, the product may become deformed or damaged. Resin gears and ring gears deform particularly easily, so please handle with care.

2. Caution on Performing Secondary Operations

1.
Gears are machined based on the ground section of the hole or shaft. If machining, it is important to pay special attention to locating the center in order to avoid runout. (Fig.1)
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 hub end surface.

Lathe Operations

2.
If enlarging the bore diameter, the wall strength of the hub must be higher than that of the gear teeth. The maximum bore size should be 60% to 70% of the hub diameter (or tooth root diameter), and 50% to 60% for keyway applied modifications. Also, because the cast FC200 hub is weaker and more brittle than other steels, we recommend using a maximum bore diameter about 10% smaller.
3.
As the worm wheel is casted, bubbles may form inside the material. If the air bubbles found in secondary operations are problematic, contact the supplier.

3. Points of Caution during Assembly

1.
The recommended center distance tolerance of KHK stock worm gears is H7 for ground gears and H8 for cut gears.
The amount of backlash is given in the product table for each gear.
2.
The mounting reference surface of the worm wheel is as shown in the figure below.
Assemble the worm wheel so that the center of the worm shaft is at the center of its tooth width.

Mounting Reference SurfaceSingle

3.
As the tooth trace of worm gears is spiraled, axial thrust force is generated. Also, the directions of thrust change with the hand of helix and the direction of rotation. This is illustrated below. The bearings must be selected properly to be able to handle these thrust forces. For more technical information, please see the section “Gear Forces” in our separate technical reference book.

Direction of Rotation and Thrust Force

4.
The worm may move due to a large thrust force that acts on the worm if it is not sufficiently attached to the shaft. Use a stepped shaft to secure the worm and shaft, and be careful not to loosen the bearing.

4. Confirming the installation

The wear of a worm gear is greatly affected by the quality of assembly. When assembling, confirm the following items for tooth contact and the like before use. For more technical information, please see the section “Worm Gear Tooth Contact” in our separate technical reference book.

The shaft angle tolerance between the worm shaft and worm wheel shaft is 90°±1’.

The tolerance between the center of the worm shaft and center of the worm wheel tooth width is ±0.2 mm.

The center distance tolerance recommended by the worm gear assembly distance is Ground Worm Gears…H7 / Cut Worm Gears…H8.

The tolerance between the center of the worm wheel shaft and center of the worm tooth width is ±2mm.

5. Cautions on Starting

1.
Check the following items before starting.
- Are the gears installed securely ?
- Is there uneven tooth contact ?
- Is there adequate backlash ? (Be sure to avoid zero-backlash.)
- Has proper lubrication been supplied ?

2.
If gears are exposed, be sure to attach a safety cover to ensure safety. Also, be careful not to touch rotating gears.

3.
For more technical information on lubricating gears, please see the section “Gear Lubrication” in our separate technical reference book.

4.
If there is any abnormality such as noise or vibration during startup, stop the operation immediately and check the assembly condition such as tooth contact, eccentricity and looseness. For more technical information, please see the section “Gear Noise and Countermeasures” in our separate technical reference book.

KHK considers safety a priority in the use of our products.
When handling, adding secondary operations, assembling, and operating KHK products, please be aware of the following issues in order to prevent accidents.

caution mark Warning: Precautions for preventing physical and property damage

When using KHK products, follow relevant safety regulations (Occupational Safety and Health Regulations, etc.).
Pay attention to the following items when installing, removing, or performing maintenance and inspection of the product.

Turn off the power switch.
Do not reach or crawl under the product.
Wear appropriate clothing and protective equipment for the work.

caution mark Caution: Cautions in Preventing Accidentse

Before using a KHK product, read the precautions in the catalog carefully in order to use it correctly.
Avoid use in environments that may adversely affect the product.
Our products are manufactured under a superior quality control system based on the ISO9000 quality management system; if you notice any malfunctions upon purchasing a product, please contact the supplier.