How to Set Worm Ratio and Holding Torque for Drive Slewing Units

Drive slewing units combine a rotating bearing, a worm-gear drive, and a housing into one assembly that turns a load in a controlled way and holds it in position. Most product pages describe the construction and the heavy-load applications. They say little about the decisions a designer must make first: which worm ratio to specify, how that ratio affects output speed and torque, and how much holding torque the application needs. This guide covers that preliminary sizing logic so you can narrow your configuration before requesting quotations.

Why Worm Ratio and Holding Torque Must Be Chosen Together

In a worm-driven slewing assembly, the worm ratio is the number of worm revolutions needed for one revolution of the output ring. A higher ratio gives more torque multiplication and slower output speed. It also usually lowers mechanical efficiency and raises the chance that the drive will resist back-driving.

Holding torque is the torque the drive can resist at standstill without the load rotating the output. It depends on the gear geometry, friction, the strength of the gear teeth, and any brake on the motor. Because ratio affects all of these, the two parameters cannot be chosen independently. A ratio picked only for speed may leave the unit unable to hold the load. A ratio picked only for self-locking may make the cycle time too long.

Two limits are often confused:

• Drive torque (output torque): the torque available to rotate the load.
• Holding torque: the torque the drive can resist when stationary, which is often limited by gear strength, a brake, or both.
• Tilting moment and axial or radial load capacity: bearing limits that are separate from the gear drive. A drive with plenty of torque can still be overloaded at the bearing.

Understanding the Relationship Between Ratio, Speed, and Torque

Three basic relationships support preliminary sizing:

• Output speed (rpm) = input speed (rpm) ÷ worm ratio
• Output torque = input torque × worm ratio × efficiency
• Required input torque = required output torque ÷ (ratio × efficiency)

Efficiency is the part that surprises many designers. Worm gearing runs with high sliding friction, so efficiency is often much lower than in spur or planetary stages. Low-lead, high-ratio worms are generally less efficient than low-ratio ones. Efficiency also differs between starting and running, and it changes with temperature, lubrication, and surface finish. Treat any efficiency figure in your first calculation as a placeholder and confirm it against the manufacturer's data.

The self-locking question

A worm drive tends to self-lock when the lead angle of the worm is small enough that friction prevents the wheel from driving the worm. In practice, self-locking is a tendency rather than a guarantee. Vibration, shock loads, wear, and lubrication can all allow back-driving, particularly with wind or inertia loads. If a load falling or drifting would be a safety issue, specify a holding brake or another independent means of holding the load rather than relying on self-locking alone.

Side-by-Side Worm Ratio Selection Table

The table below compares generic ratio bands at an assumed input speed of 1,500 rpm. The ratios are illustrative and do not describe any specific product. Actual available ratios, efficiencies, and locking behavior vary by manufacturer and model.

Read the table from left to right as a tradeoff. Moving to a higher ratio buys torque and holding behavior but costs speed and efficiency. The higher friction also becomes heat, so continuous-duty applications need a check on thermal limits as well as torque.

A Step-by-Step Sizing Method for Preliminary Selection

Step 1: Define the load and the motion

Record the axial load, radial load, and tilting moment at the bearing. Then define the rotation task: the angle to travel, the required cycle time, the duty cycle, and whether the load is balanced. Separate the bearing check from the drive check, because each has its own limits.

Step 2: Estimate the required output torque

Add the torque needed to overcome friction, the torque to accelerate the load inertia, and any torque from external forces such as wind or an eccentric load. Add a safety factor suited to the duty. Shock, reversing loads, and outdoor exposure justify a larger margin.

Step 3: Set the target output speed

Many slewing applications run at only a few revolutions per minute or less. Work backward from the travel angle and cycle time to find the required output speed. Include acceleration and deceleration time so the average speed is not understated.

Step 4: Choose a ratio and check the input

Divide the motor speed, or the speed after any upstream gear stage, by the target output speed to find the ratio you need. Compare that with the ratios actually offered. Then use the efficiency estimate to check the input torque the motor must supply.

Step 5: Verify holding torque

Compare the static torque from the load with the holding rating of the drive and any brake. Check both normal conditions and worst-case conditions such as peak wind, emergency stops, and shock. If the load can back-drive the unit, add or upsize an independent brake.

Worked Example With Illustrative Numbers

Assume a platform needs 3,000 N·m of output torque, including a safety factor, and a rotation speed of about 1 rpm. The motor runs at 1,400 rpm.

• Total reduction needed: 1,400 ÷ 1 = 1,400:1.
• Split the reduction: a 20:1 planetary gearbox ahead of a 70:1 worm stage gives 1,400:1.
• Assume a worm-stage efficiency of 35 percent as a placeholder.
• Required torque at the worm shaft: 3,000 ÷ (70 × 0.35) ≈ 122 N·m.
• With the 20:1 planetary stage at an assumed 95 percent efficiency, motor torque ≈ 122 ÷ (20 × 0.95) ≈ 6.4 N·m.

This is a screening calculation only. It shows that the worm ratio and the upstream stage share the reduction, and that low worm efficiency drives up input torque. The final selection should rely on manufacturer-rated torque, efficiency data, and a thermal check. If a holding brake is fitted at the motor, remember that the brake sees the load torque divided by the total ratio and efficiency, not the full output torque, so it can be much smaller than the output holding requirement.

Common Specification Mistakes

• Treating self-locking as a safety feature. Vibration and wear can reduce locking. Use a brake where a drifting load is hazardous.
• Ignoring efficiency. Using 90 percent efficiency in a worm calculation can undersize the motor several times over.
• Confusing bearing capacity with drive capacity. The slewing bearing and the worm drive have separate limits, and both must pass.
• Overlooking duty cycle. Low-efficiency, high-ratio drives generate heat, which matters in continuous or frequent-start operation.
• Specifying peak speed without acceleration time. This can leave the unit unable to meet the cycle.
• Leaving out backlash and positioning accuracy. If the application needs fine positioning, ask about backlash, preload, and control resolution.

What to Include When Requesting a Quotation

A complete request helps a supplier recommend the right unit the first time. Include:

• Axial load, radial load, and tilting moment, with peak and operating values.
• Required output torque, holding torque, and output speed.
• Motor type, supply voltage, and whether a brake is needed.
• Duty cycle, ambient temperature, and exposure to dust, water, or corrosion.
• Mounting interface, envelope limits, and any sensor or encoder requirements.
• Expected service life and maintenance access.

The bearing side of the assembly deserves the same attention. For related guidance, see this article on selecting rotary table bearings for CNC machines, which covers load and stiffness concerns that also apply to rotating assemblies.

Finding Suppliers and Comparing Options

Once your ratio band and holding requirement are set, the next step is comparing suppliers on rated torque, available ratios, brake options, and documentation. You can browse listings in the machinery category to find manufacturers and submit your technical requirements. If you want to compare how other buyers evaluate suppliers, you can also find suppliers on link and review the information they provide. Always request datasheets and confirm rated values directly with the supplier, since the figures in this article are illustrative.

Summary

Setting the worm ratio and holding torque is a balancing exercise. Lower ratios give faster output and better efficiency but weaker locking. Higher ratios give more torque multiplication and stronger holding tendency but slow the rotation and generate more heat. Start with the load and cycle time, calculate the ratio from the speeds, correct for realistic worm efficiency, and verify holding torque against worst-case conditions with a brake where safety requires it. With these numbers in hand, your selection of drive slewing units will be far more focused, and your supplier discussions will be quicker and more precise.