Doubts about the operation of a clutch 2

[Galvano]

Good afternoon,
Sorry for my bad English.

At my work, there is a machine that has two reels of tape. The machine has two axes that collect the tape and wind it on one reel or another.

The coils move in one direction or another by means of a transmission with clutches. These clutches are cork.

The question I have is whether these cork clutches have the same torque in one direction as in another. They are made up of two cork discs, the transmission pinion and a spring that compresses it. The question is... Is the torque the same in one direction as in another?

Thanks

 

[3DDave]

How close do they have to be to be "the same"? I doubt they will be exactly equal, but the maximum friction should be fairly close.

Is there a problem you are trying to solve?

 

[Galvano]

For example, if in one direction of rotation the necessary torque is 0.1 Nm, in the other direction, how much could it be? I understand that it should be the same and if it is not, why?


With the spring the force is regulated, but since the pinion is made of plastic, the cork slides quite a bit. I suppose that putting some metal discs on the faces of the pinion would improve the adherence. Or maybe put a stronger spring?

The corks are polished with the turn but it is not a good system as the pinion is made of plastic.

Thanks

 

[desertfox]

Hi Galvano

I can’t see why there would be a significant difference between rotation in either direction, what makes you think there will be ?



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Galvano]

Hello,

Thanks for the reply. As I say, it is a machine that has two axes like the one in the photo and in one of which a reel of tape is placed and in the other a cylinder to collect it.

In one direction of rotation, it winds the tape on one coil and in the other on the opposite.

So let's say that when one coil pulls on the tape, the other has to let go. One clutch pulls and the other slips. When it turns the other way, the same thing happens. That's why I thought that in one direction a clutch allowed more torque than in the other.

 

[Galvano]

The problem is that the cork slips on the plastic pinion when it is used.

Would it be better to put a stronger spring or place some metal discs screwed on both sides of the pinion?

I have seen that in this type of clutch the cork makes contact with steel. I suppose that the friction between cork and steel is greater than between cork and nylon.

 

[desertfox]

One difference in the torque would be the inertia of the larger roll of tape on one axis at the start up of the winding operation. However you haven’t shared any details about the tape that is transferred from one to the other.

Looking at the photo you supplied the friction is generated mainly by the compression spring which can vary in spring rate depending on the class it is manufactured in accordance with but you could compress two of the same spring and have a difference in spring force between 10 and 20% and that’s before we even talk about sliding friction coefficients between cork and nylon.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Galvano]

So, to avoid unwanted slippage of a clutch in the tape winding operation, do you recommend increasing the spring force or putting a metal disk on the faces of the nylon sprocket?

The cork loses adherence with use but I think the system is not good because the nylon is very slippery and the springs are not very strong. Is there a way to calculate the spring force needed to prevent slippage?

It is necessary to regulate the clutches on both sides so that it works in both directions.

 

[3DDave]

This sort of clutch is intended to slip, generally to prevent damage from overload. If the slipping is unwanted glue the parts together and see what breaks. That will find out what the actual problem is.

 

[desertfox]

Hi Galvano

Yes you can calculate the spring force that’s fairly straightforward we just need the free length of the spring, the compressed length of the spring when preloaded and wire diameter, number of total coils, outside diameter of the spring for starters. Without physical sizes and material properties of the cork and nylon the spring force is pretty useless, we also need the operating torques of the shafts being driven by whatever motor/driver so that when you give us the coefficients of friction for the cork , steel and nylon we can set a spring force that will prevent slipping to a certain point.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[3DDave]

Something seems odd about the design - the outer cork piece should have a substantial metal plate behind it so the load is uniformly applied, but the photo is too poor quality to show it if there is one.

If you don't want it to slip cut out the middle of that cork so there is only a rim contacting the gear - this will increase the leverage but it still needs a substantial metal plate to work that way.

 

[Galvano]

3Dave yes, both corks are glued to a steel plate. the spring pushes on the steel plate.

My question is whether to increase grip, it would be better to put a stronger spring or glue some steel discs on both sides of the pinion so that the cork would be made of metal instead of nylon and thus increase grip.

As the other colleague says, I will put the data of the spring and the diameter of the cork discs to calculate the torque obtained by compressing the spring completely.

 

[Galvano]

Desertfox

The data is:
Spring outer diameter: 8 mm
Overall Length: 16.5mm
turns: 7.5
Spiral diameter: 1 mm
Compressed spring length: 10 mm

Cork disc diameter: 35 mm

I read that the coefficient of friction of cork is 0.6 but being in contact with nylon I don't know if it is valid.
The coefficient of friction of nylon read that it is 0.1 to 0.3

With these data, do we know the maximum torque that the clutch can withstand without slipping?

 

[desertfox]

Hi Galvano

I get a spring force of 34 N assuming the spring is made from carbon steel and if I assume a coefficient of sliding friction of say 0.3 then the torque before slipping would be 35.7Nmm. I assumed that the spring force acted on the cork/nylon at the mean diameter of the spring which would be 7mm, which yields a radius of 3.5mm and so:-

0.3 x 3.5mm x 34N = 35.7Nmm

Now here is the difficulty in knowing what the coefficients of friction actually is because if I assume 0.6 for the friction coefficient, then it doubles the slipping torque, you would need to do practical tests and measurements on the actual device to fine tune all the components.
A further complication is the more you compress the spring the more stressed it becomes and eventually the spring can get over stressed the more you compress it.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Galvano]

Cool,

The exact coefficient of friction cannot be known since it is a combination between cork and nylon. So it's ok to consider 0.3

According to your calculations, the torque obtained is obtained by the radius of the spring, not the cork disk, which is only compressed at the central point.

Given this, and since I don't know the minimum torque necessary for the clutches not to slip in any direction (when winding or unwinding), should I increase the spring force or increase the diameter of the spring? Increasing the force is easier than increasing the diameter.

On the other hand; What would be the maximum torque that the cork would hold without breaking? It is a cork approximately 1 mm thick. He thought of a spring with the same data but with a coil diameter of 1.2 mm

 

[desertfox]

Hi Galvano
Well I can’t really answer that because we need to know the operating torque when the tapes are winding or unwinding, most of the friction will be located around the area of the spring, the load spread over those discs will probably fall to zero way before the outer diameter of either the cork or nylon. Well the most you can adjust the spring is 2.5mm before the coils contact each other and act as a solid.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Galvano]

Of course, the minimum and maximum diameter of the coils I can know. When one increases the diameter the other decreases.

I think the problem is that apart from not being a good system because the nylon is very slippery, the cork is polished and the springs only exert pressure at the central point, so with a spring with a larger diameter or stronger, the torque transmitted would be higher.

I suppose that changing the corks for new ones would continue to happen the same. In any case, when mounting the assembly, the springs do not exert as much pressure. I do not see that they are well dimensioned.

 

[desertfox]

Hi Galvano

If you increase the OD of the spring only it will decrease the spring force for the same compression, reducing the OD will increase the spring force but when you alter any spring you need to check the stress in the spring itself. If your clutches are slipping in operation then it will cost you nothing to compress the spring in the assembly a little bit more but I have no idea whether it will work or not but it’s worth a try, if that doesn’t work then you need to get more information about the system itself in terms of the axis driver torque and mass of tape on each spool etc then and only then you might be able to correct the problem. Changing the cork or spring force and or nylon could be an endless loop of effort.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[3DDave]

Who designed this?

 

[Galvano]

Yes desertfox. When it slipped in one direction I tightened the nut to prevent it from doing so, but then in the other direction of rotation it did slip.

That is why I said putting stronger springs to be able to compensate for the possible poor dimensioning of the mechanism and the wear of the corks, which, being in contact with nylon, slide more than, for example, if they were with steel.