... compared to regular cog wheel drives.
Introduction
This article is intended to provide a true technical explanation to why crawlers with worm drives are slower than other crawlers with same motor. In the process it will defy one or two misconceptions too often expressed in this forum.
Background
I’ve got a MSc in engineering and was very curious to find out why my LCC (which I bought half a year ago) didn’t run as well as I thought it should. The all too common explanation, “Worm drives provide a huge gear-down, therefore they’re slow!”, just isn’t true (as is obvious to anybody with mechanical knowledge). The also common comment, “Worm drives eat brushed motors!” seemed to have more truth behind it, since my motors got very hot. (*)
Obviously the worm drives impose drag and power loss, but how much?
The answers
First let’s kill the gear-down misconception:
- Worm drives do have a bigger drive shaft to wheel axle ratio. That much is correct, but...
- Worm drive crawlers compensate that, in full, by having a lesser motor to drive shaft ratio.
- For a given motor speed the wheels will therefore turn about as fast no matter if there are worm drives or not!
So why is it in reality so that worm drive crawlers are slower?
- To move the crawler takes mechanical power. The power is equal to torque times rotation speed (“angular velocity”, is the correct term).
- The higher the speed of the car, the more power must be provided to the wheels. (Same or more torque times more speed.)
- While providing power to the wheels, the motor also has to overcome some power loss within the drive train. Here’s the key issue: The power loss is far greater with worm drives than with cog wheels, so for a given motor power more of it is consumed by the drive train and less reaches the wheels, hence less vehicle speed can be reached! (All of that consumed motor power is transformed into heat, causing the worm drive to get warm and eventually hot with risk of overheating!)
Why are worm drives so inefficient?
I found this very informative home page that explains it in detail.
The main issue is friction.
- Power transfer between cog wheels is a rolling motion with very little friction. Most of the resistance is between each cog wheel and its axle.
- Power transfer in a worm drive is a sliding motion with the pinion (“screw”) sliding sideways against the gear cogs. This cause considerable friction between pinion and gear.
I applied some of the equations to the worm drives in my LCC, using approximate numbers for angles and friction.
The result was very sobering, to say the least.
Given the relatively slow sliding speed (<1 m/s at 1,500 rpm of the drive shaft) and less than optimal lubrication (grease instead of an oil bath) the friction coefficient stay above 0.1.
The screw angle is only about 10 degrees with the standard drive, and somewhat more with the HD drive. With a friction coefficient of 0.15 this result in an efficiency as low as about 50% for the standard drive! (Slightly better with the HD, and also better (but not good) at higher speeds.)
Neglecting all other losses in the drive train, which is a fairly good approximation by the magnitudes, that means an LCC will need a motor roughly twice as powerful as that of an AX10 to reach the same speed!
It’s also worth noticing that at very low speeds the power is low, so the absolute loss in power also stays low. Here the emphasis is on raw (motor) torque. Therefore worm drives do well for low speed use.
Combining these findings it becomes clear that building a rock racer with worm drives is a non-starter:
- High speed requires lots of power, so a high power motor is required.
- The worm drives will consume a fair bit of the delivered power, so even more power is needed from the motor to keep up with the competition.
- All that power will make both the motor and worm drives to run much hotter than the non-worm equivalents, and heat is THE limiting factor.
Late addon:
So why use worm drives at all?
There are a couple of advantages to using worm drives in crawlers:
(*) The worm drives wasn’t my main problem, initially. I made a couple of mistakes mounting the motor and wheel axles that made up for most of the difficulties.
Introduction
This article is intended to provide a true technical explanation to why crawlers with worm drives are slower than other crawlers with same motor. In the process it will defy one or two misconceptions too often expressed in this forum.
Background
I’ve got a MSc in engineering and was very curious to find out why my LCC (which I bought half a year ago) didn’t run as well as I thought it should. The all too common explanation, “Worm drives provide a huge gear-down, therefore they’re slow!”, just isn’t true (as is obvious to anybody with mechanical knowledge). The also common comment, “Worm drives eat brushed motors!” seemed to have more truth behind it, since my motors got very hot. (*)
Obviously the worm drives impose drag and power loss, but how much?
The answers
First let’s kill the gear-down misconception:
- Worm drives do have a bigger drive shaft to wheel axle ratio. That much is correct, but...
- Worm drive crawlers compensate that, in full, by having a lesser motor to drive shaft ratio.
- For a given motor speed the wheels will therefore turn about as fast no matter if there are worm drives or not!
So why is it in reality so that worm drive crawlers are slower?
- To move the crawler takes mechanical power. The power is equal to torque times rotation speed (“angular velocity”, is the correct term).
- The higher the speed of the car, the more power must be provided to the wheels. (Same or more torque times more speed.)
- While providing power to the wheels, the motor also has to overcome some power loss within the drive train. Here’s the key issue: The power loss is far greater with worm drives than with cog wheels, so for a given motor power more of it is consumed by the drive train and less reaches the wheels, hence less vehicle speed can be reached! (All of that consumed motor power is transformed into heat, causing the worm drive to get warm and eventually hot with risk of overheating!)
Why are worm drives so inefficient?
I found this very informative home page that explains it in detail.
The main issue is friction.
- Power transfer between cog wheels is a rolling motion with very little friction. Most of the resistance is between each cog wheel and its axle.
- Power transfer in a worm drive is a sliding motion with the pinion (“screw”) sliding sideways against the gear cogs. This cause considerable friction between pinion and gear.
I applied some of the equations to the worm drives in my LCC, using approximate numbers for angles and friction.
The result was very sobering, to say the least.
Given the relatively slow sliding speed (<1 m/s at 1,500 rpm of the drive shaft) and less than optimal lubrication (grease instead of an oil bath) the friction coefficient stay above 0.1.
The screw angle is only about 10 degrees with the standard drive, and somewhat more with the HD drive. With a friction coefficient of 0.15 this result in an efficiency as low as about 50% for the standard drive! (Slightly better with the HD, and also better (but not good) at higher speeds.)
Neglecting all other losses in the drive train, which is a fairly good approximation by the magnitudes, that means an LCC will need a motor roughly twice as powerful as that of an AX10 to reach the same speed!
It’s also worth noticing that at very low speeds the power is low, so the absolute loss in power also stays low. Here the emphasis is on raw (motor) torque. Therefore worm drives do well for low speed use.
Combining these findings it becomes clear that building a rock racer with worm drives is a non-starter:
- High speed requires lots of power, so a high power motor is required.
- The worm drives will consume a fair bit of the delivered power, so even more power is needed from the motor to keep up with the competition.
- All that power will make both the motor and worm drives to run much hotter than the non-worm equivalents, and heat is THE limiting factor.
Late addon:
So why use worm drives at all?
There are a couple of advantages to using worm drives in crawlers:
- Slim pumpkin.
- Increases the ground clearance under the axle. - Pinion axle above the wheel axle.
- More clearance under the drive shaft.
- The drive shaft is straighter, causing less friction in the joints. - Built in drag brake. (Not adjustable though.)
- Large amount of end gearing, in a compact format.
- Reduced gearbox size/weight.
- Less torque on the drive shaft and therefore very little torque twist.
(*) The worm drives wasn’t my main problem, initially. I made a couple of mistakes mounting the motor and wheel axles that made up for most of the difficulties.
Last edited:
