What is the best gear ratio between PF motor and 24T Clutch Gear?

[hrontos]

I wonder, if there is any official TLG guide/rule or somebody has done some experiments with proper placement of 24T clutch gear in a drive train for each PF motor.

For example, for PF XL motor, geared up 1:2. Will it have enough torque to activate the clutch in the 24T gear without stalling or putting too much load on the motor?

I would like to know this optimal gear ratio for each PF motor.

To avoid confusion I would like to know the gear ratio between axle of motor and axle of the 24T Clutch gear. I know it may depend also on friction of the drive train between these two axles, but let assume there is none.

May be others will find it also useful just like the other parameters of the motors.

Is TLG calculating the placement of the 24T clutch gear in a drive train or they are just long term testing the model?

[Zerobricks]

Depends...first of all there are different 24t cluctch gears. The older ones had I think 2,5 - 5Ncm written on them. The newer ones are much harder, but their indiviudal friction values can differ a lot... I think the best way is simply to take a few gears and keep stacking them on motor untill it stalls...Than you know your limit. But remember the state of batteries also has a lot of effect on the torque of the motors.

Edited July 17, 2014 by Zblj

[hrontos]

I think the best way is simply to take a few gears and keep stacking them on motor untill it stalls...

So from the motor point of view, it is not important and will not cause any harm to the motor, as long as it is not completely stalled?

I mean, when a maximum gear ratio is found, it is not important how much load it puts on the motor, how much it will struggle to activate the clutch, as long as it is not stalled?

PF XL produces something like 25Ncm, so even geared up 1:5 should be able to active the older clutch gear without problems. May be even 1:10.

Correct placement of the clutch gear is not important when load is not high and there is only an "end of travel" risk - like with LA.

But when load is high, correct placement of the clutch gear is important, because it protects the motor from stalling due to heavy load and also determines maximum load motor will handle.

When it is "too late" in the drive train, max. power of the motor is not fully used, because clutch activates, when "too early", motor may have not enough power to activate the clutch and will get stalled.

I though TLG has some kind of rule for this.

Edited July 17, 2014 by hrontos

[Splat]

I have also wondered this in the past. It may be worth looking at official Lego sets that have this piece to see where it is placed.

In addition to the gear-train between the motor and the 24T Clutch Gear, I think it is also important to know what is on the other side of the clutch gear. If you have further gearing after the clutch gear, this will also determine at which point the gear starts to slip.

For example, if it is used in a car, you don't want the clutch gear to slip as soon as the car hits a little bump, but you do want it to slip if it hits a wall or other impassable object.

[hrontos]

In addition to the gear-train between the motor and the 24T Clutch Gear, I think it is also important to know what is on the other side of the clutch gear. If you have further gearing after the clutch gear, this will also determine at which point the gear starts to slip.

For example, if it is used in a car, you don't want the clutch gear to slip as soon as the car hits a little bump, but you do want it to slip if it hits a wall or other impassable object.

When I was thinking, if the drive train after the clutch gear is important, I took it this way: when it is necessary to get maximum of the motor, the structure of the drive train after the clutch seems to be not important.

I assumed, it is needed to get max out of the motor. This means a car that is able to climb any reasonably sized obstacle with maximum speed (that is possible with given motor and weight of the vehicle). Hitting a wall is like too big obstacle and that's exactly case, when the clutch is needed. But how to find a maximum gear ratio for the clutch gear? Trial/error approach works as long, as the drive train is simple. But for the complex one, the rebuild is tricky and it would be better to place the clutch properly from the beginning.

It is not important, if the goal is to move car quickly or lift a heavy weight. They both need a certain speed or torque. And clutch gear is able to transfer only constant maximum torque. After the clutch gear, the torque can be increased or decreased, but starting point are those 2.5-5Ncm. That's the maximum torque, that will get through clutch gear, so it will look like torque limiter. But speed is not limited. So to get more, motor has to be geared up to have extra speed at the clutch gear, that can be after the clutch gear changed to torque.

For example: if PF XL has really 25Ncm, 1:1 attachment of the clutch gear means, that after the clutch gear, we have only those 2.5-5Ncm supported by the clutch gear. Speed is 1:1 so rest of the motor power is lost.

But if PF XL is geared up 1:10 before the clutch, we have extra speed. We get only 2.5-5Ncm after the clutch gear, but 10times higher speed. So after gearing it down 1:10 after the clutch, we get again 25Ncm as on the motor (virtually full motor power), but the motor and whole drive train is protected. If it is a car, it will climb also big obstacles, but nothing will be damaged when it hits a wall.

The only question is, if it should be 1:10, 1:6, 1:20 or which value, to get maximum of the motor and have it protected, just like TLG does.

Edited July 17, 2014 by hrontos

[Blakbird]

I don't think the clutch gear is really there to protect the motor from stalling. Stalling the motor is not that big of deal although it does use a lot of power. Particularly with an XL motor, the clutch is there to protect the model from the forces generated by a stalled motor.

[hrontos]

I don't think the clutch gear is really there to protect the motor from stalling. Stalling the motor is not that big of deal although it does use a lot of power. Particularly with an XL motor, the clutch is there to protect the model from the forces generated by a stalled motor.

That's a good point. This means, that even with a clutch gear, when geared down too much after the clutch gear, the drive train after the clutch gear can suffer similar high forces like unprotected drive train?

[CP5670]

Depends...first of all there are different 24t cluctch gears. The older ones had I think 2,5 - 5Ncm written on them. The newer ones are much harder, but their indiviudal friction values can differ a lot... I think the best way is simply to take a few gears and keep stacking them on motor untill it stalls...Than you know your limit. But remember the state of batteries also has a lot of effect on the torque of the motors.

There is even a third variety, from the original 1997 sets when they first came out with the clutch gears. Those have the same markings but a light gray center, and I think are even harder than the current ones.

[Blakbird]

That's a good point. This means, that even with a clutch gear, when geared down too much after the clutch gear, the drive train after the clutch gear can suffer similar high forces like unprotected drive train?

That's right. You have to pay attention to where in the drive train you put the clutch if you want it to protect anything. They were originally used in 8479 where the arm and dumper were geared down hundreds of times to one. If not for the clutch gear, the potential torques would have destroyed parts. Plus, that model ran the motors on a timer so the output was guaranteed to stall for a while once it hit the stop until the timer shut off the motor.