Clutch stiffness and hysteresis - Role in gearbox NVH

[k.udhay]

Hi,
There is a rattling noise in one of the automobile passenger car gearboxes that we designed. After few experiments and observations, it was found that the noise is because of incorrect clutch stiffness and hysteresis behavior. I have got some fundamental questions about both:
1. Clutch stiffness Vs rattling:

• I understand that higher the stiffness of clutch, lower the relative displacement between crank shaft and transmission shaft for a particular torque.
• All infiltrated fluctuations to the clutch will be absorbed to an extent. And this is done by the springs in the clutch. But, won't the springs start vibrating (as it is an elastic body and not a dampening member) after they absorb this fluctuating forces? What is better for rattling - High stiffness clutches or low stiffness clutches?

2. Clutch hysteresis Vs rattling:

• The attachment shows the hysteresis values of the clutch that I am discussing here about.
• The unit is NM. What exactly does this hysteresis torque physically mean?
• How is hysteresis responsible for gearbox rattle?

Thanks a lot!

 

[Danger]

I've turned a lot of wrenches on a lot of cars, but I've never even heard of the stuff that you're talking about. To me, a rattling sound would indicate a loose part or a worn bearing. Sorry.

 

[xxChrisxx]

It's not anything worn. But the OPs is a tricky question, as it's fairly high level NVH. It's going to be very hard to give a detailed answer.

Rattle is caused by rapid impacts due to lash in the gears. Excited by the torsional vibration of an engine.

A sprung clutch allows a certain degree of 'free play' when torque is not being transmitted. Eg clutch up in N at idle.

Edit. Its probably easier to use a linear analogue. Think of a car suspension.

The torque transfer (ie opening the throttle) is like large hills. The entire car moves over them together. So the springs don't do much.

Torsional vibrations are a rough road surface. If you had no suspension then you would feel all the little bumps. Adding a spring to a clutch isolates the driveline from TV like suspension springs isolates you from those little bumps in the road. It's a bit of a butchered explanation.

 

[Danger]

It's a bit of a butchered explanation.

Not bad, considering the subject matter. The way you describe it, though, it sounds more like clutch "chatter" that we used to experience in my day. That was caused by either misalignment, a bad pilot or throwout bearing, or worn disk material. I've never heard anything like that caused by lash, but maybe that's just because I've never encountered lash. It only occurred during engagement/disengagement. The impression that I got from the OP was that the phenomenon in question was constant. Can you provide a touch more explanation, without getting into math?

 

[xxChrisxx]

It's fairly difficult to distinguish subtleties between similar noises using subjective phrases. Gear rattle sounds sort of similar to a knackered release bearing, but a bit more 'clacky'.

The loose gears sit on the needle bearings, when the small torque changes due to torsional vibration exceed the drag in the bearings then the gear is free to rotate. This is when you have a risk of gear rattle.

As you describe clutch 'chatter' only occurring during engagement and disengagement. Does it come across as a jerking? As it sounds similar to what we would describe as 'stick-slip'.

 

[Danger]

Gear rattle sounds sort of similar to a knackered release bearing, but a bit more 'clacky'.

Non-gearheads might find it hard to believe, but that makes perfect sense to me.

Does it come across as a jerking?

Close, but not exactly. More like sliding down a washboard on your ass.

what we would describe as 'stick-slip'

Maybe. I'm not familiar with all Brit terminology since we use a combination of that and Yank-speak. For instance, we usually refer to the rear compartment as a "trunk", but sometimes it's the "boot". A vertical conveyor can be a "lift" or an "elevator" depending upon what time of day it is or how much one has had to eat. :D In other matters, we use one or the other pretty exclusively (or French of course; we're a bilingual nation).

 

[k.udhay]

Thanks, xxChrisxx... Can you suggest me a book or paper that speaks more detailed about this? I am now more curious to understand this clutch hysteresis. Thanks.