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.

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.



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)

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

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)

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
NOTE 1: Select the same module for both members.

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.

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)

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.
   
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.
   
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.

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

• Check the Mounting distance (allowable Mounting distance H7~H8).

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


Application Examples