Why is worm gear and worm shaft transmission “thankless”?

Among the slewing drives designed and manufactured by U-TRSM, worm gear slewing drives take up the vast majority. As a common mechanical transmission method, worm gear and worm shaft transmission is widely used in automation equipment, lifting equipment, machine tools, automotive steering systems and other fields due to its advantages of compact structure, large gear ratio, and strong self-locking. However, its transmission efficiency is generally low (usually 50%~90%, some even less than 50%), which has become a key problem restricting its performance.

1.Sliding friction-dominated meshing characteristics

The efficiency loss of worm gear and worm shaft transmission mainly originates from its special meshing method. Different from the rolling friction of gear transmission, the tooth surface of the worm shaft and worm gear is dominated by high-speed sliding friction during the meshing process. This sliding friction can lead to the following problems:

Large relative sliding speed: the helical shape of the worm leads to a significant tangential speed difference at the point of engagement, especially when the transmission ratio is large, the sliding speed can reach several times the linear speed of the worm, and the friction power consumption increases significantly.

Frictional heat accumulation: sliding friction will generate a lot of heat, and if the heat is not dissipated in time, it will lead to an increase in the temperature of the tooth surface, which will exacerbate the softening of the material or the failure of lubrication, forming a vicious circle.

Studies have shown that sliding friction losses account for 60% to 80% of the total energy loss in worm gear worm shaft transmission, which is the core reason for low efficiency.

2. Limitations of lubrication conditions

The lubrication state has a decisive influence on the efficiency of worm gear worm shaft transmission, but their lubrication conditions are often difficult to optimise:

Oil film is difficult to form: high-speed sliding friction leads to the lubricant is quickly extruded from the engagement zone, boundary lubrication or mixed lubrication state is dominant, the friction coefficient is significantly higher than the fluid lubrication.

Temperature rise leads to lubrication failure: frictional heat makes the lubricant viscosity decrease, the oil film carrying capacity decreases, and even triggers oxidative deterioration. For example, when the oil temperature exceeds 80℃, the lubrication performance of mineral oil will deteriorate sharply.

The lubrication method is limited: limited by the worm helix structure, it is difficult for the lubricant to cover the meshing surface uniformly, and it is easy to have insufficient local lubrication, resulting in pitting corrosion or gluing failure.

3. Contradiction between material matching and coefficient of friction

The material selection for worm gears and worm shafts needs to be balanced between strength and friction reduction needs, but they are often in conflict:

Worm gear material selection: in order to reduce friction, the worm gear is often used tin bronze (ZCuSn10P1) and other non-ferrous metals, but its hardness is low, easy to wear; If a steel worm gear is used, although it can improve the life expectancy, but the coefficient of friction will rise 10% to 30%.

Limitations of worm shaft material: worms are usually made of carburised steel (e.g. 20CrMnTi) to increase the surface hardness, but sliding friction between high hardness materials still produces significant energy losses.

Surface treatment technology: plating (such as phosphating, copper plating) or coating (DLC diamond-like coating) can reduce the coefficient of friction, but the cost is high, and it is difficult to completely eliminate the sliding friction loss.

4. Trade-off between self-locking properties and efficiency

When the helical lift angle of the worm is less than the equivalent friction angle between the tooth surfaces, the transmission system will have a self-locking function (i.e., the worm gear cannot drive the worm in reverse). Although this feature has safety advantages in scenarios such as lifting machinery, it can lead to:

Dramatic increase in frictional resistance: in the self-locking state, the normal pressure on the engaging surfaces increases and the frictional power consumption rises significantly.

Non-linear decrease in efficiency: the efficiency of self-locking worm shaft transmission is usually less than 50%, and the larger the gear ratio, the more obvious the decrease in efficiency.

Therefore, whether or not to enable the self-locking function needs to be weighed against the need for safety and efficiency in the light of actual working conditions.

5. Difficulty in heat dissipation and thermal deformation effects

The closed structure of the worm gear and worm shaft transmission results in inefficient heat dissipation, and heat build-up triggers a chain reaction:

Thermal expansion leads to deterioration of meshing: temperature rise causes uneven expansion of the worm shaft and worm gear, reducing the meshing gap and further increasing friction.

Risk of lubricant carbonisation: high temperatures may cause the lubricant to coke, forming hard particles that accelerate tooth surface wear.

Degradation of material properties: bronze worm gear at 150 ℃ above the yield strength decreased by 30% ~ 50%, exacerbating the risk of plastic deformation.

6. Other influencing factors

In addition to the above core reasons, the following factors can also indirectly reduce transmission efficiency:

Manufacturing and assembly errors: worm shaft guide angle deviation, worm gear tooth shape error, etc. will increase the local contact stress, resulting in additional friction.

Working conditions: low-speed and heavy-duty conditions are prone to boundary lubrication, while high-speed conditions result in insufficient lubricant splash due to centrifugal force.

In summary, the efficiency constraints of worm gear and worm shaft transmission are the result of the cross effect of tribology, material science, thermodynamics, and need to be optimised from the system level to break through the performance bottleneck.