VELVET TRANSMISSION NOISE

[avazquez]

We went out this weekend and when placing the transmission in forward gear it made a "clonk" noise. Then it engaged and operated normally. It did it everytime the gear was engaged and left in the lowest possible speed. But if the lever is pushed in one movement a bit further down it would not make the noise. Could it be that the cable slipped a litle bit so that it is not engaging properly?

[Alan Wheeler]

The transmission does not engage "mechanicaly" it is hydraulic pressure. The cable simply turns an internal valve that allows oil to flow to engage the internal plates that turn the gears. These boxes will go clunk if you engage at to high an RPM or throw the lever fwd to quickly. Ensure you are at idle before you apply fwd. If you are moving fwd and want to apply rvrs, then do so softly. Do not throw the lever back, as the prop is still spinning in the water flow.
To check all is OK, First thing I would look at is ensure all bolts are tight on the drive coupling and that the coupling has no slop in it. Ensure the coupling is tight on the shaft. Ensure the fluid is clean and at the proper level.Ensure the prop is tight on the shaft.

[Inthewind]

The Borg Warner Velvet drive needs to be to be engaged at idle or they will go in gear with a harsh clunk like Alan said. The only other thing I can think of is if a spring broke or broke loose on the pressure plate, that could possibly cause a clunk noise.

[Alan Wheeler]

Doesn't have a pressure plate. It's a hydraulic box.

[Inthewind]

I understand that Alan, but doesn't the transmission spline shaft stab into a pressure plate that is bolted to the fly wheel ?

[Alan Wheeler]

Not in this case. It's a little different. It's quite a cool box design actually. Very simple once you get your head around how it works. Not a great deal of moving parts and that is what makes it such a reliable box.
There are three main components. The input shaft assembly, the intermediate gear assembly and the output shaft assembly. Inside the Input shaft assembly and the intermediate gear assembly are clutch packs. These are very different to a pressure plate clutch system between an engine and gearbox in say a car. the packs are similar to what is in an autotransmision box in a car. The friction plates, of which there are 8 I think, spin in an oil bath slipping over slip plates. There are teath on the outer edges of the Friction plates that lock into an outer casing and the slip plates have teath on the inner edge that lock on the Input shaft. When a gear is selected, oil pressure forces a piston to squezze these plates together and they grab the slip plates and the shaft spins the outer housing turning another gear. Dependign on which set of plates are locked determins which gear spins thus causing the output shaft to turn left or right.
There ya go, more than ya probably wanted to know.

[SkiprJohn]

There is a plate which connects to the crankshaft of the engine and then accepts the splines on the end of the velvet drive. I think its called a drive disc. I can't be certain about the name of it. If it fails after years of service it leaves the engine crank spinning and not connected to the forward end of the velvet drive. It has several springs around the outside and is designed to take some of the shock out of engaging and disengaging the gear.
Kind Regards,
JohnL

[avazquez]

God do I feel like a fool.......Sorry Sorry Sorry......It is not a Velvet It is a HURTH.................................

[SkiprJohn]

Oops. Don't feel bad. Now you know everything you don't need to know about a Velvet Drive.
Kind Regards,
JohnL

[Maddog]

While we are on velvet drives. Any idea why my shaft keeps spinning slowly in nuetral?

[S/V Elusive]

Originally Posted by Maddog

While we are on velvet drives. Any idea why my shaft keeps spinning slowly in nuetral?

Because you are sailing??

[Alan Wheeler]

Yes Maddog. Firstly I am assuming that this is in neutral at the dock or in other words not moving, with engine idling.It is just due to little friction in shaft/prop drive. Because the box itself is full of oil, there will always be some resistance to motion. The resistance is transmitted from the plates to the slip rings and thus the gear is turned. Nothing to worry about at all. It should not cause wear to bearings as the friction is so low.

[Alan Wheeler]

OK, so being a Hurth, John is correct. There will most likely be a plate between the box and the engine. This is not always used on hydraulic boxes, although can be as it is the easiest way to connect the engine flywheel to the input spline of the box. But will normaly always be used for Geared boxes for the following reason. The plate can be called many names, such as "Drive plate", "Coupling Plate" but the proper name is the "Dampner Plate". The main job of this plate is to reduce a hammering effect from Diesel Engines, called "injector shock". This "hammering" effect rattles the snot out of the internal gears of the box and can cause undue wear. I don't know if it could cause the clunk when placed in gear, but hey, I just don't know. With the Hurth box, there are clutch plates directly coupled to the gears. These plates are mechanicly operated as opposing to the Hydraulic box. The concept is similar in both boxes, but the design and way it happens is very different. I suspect that the plates maybe grabing. This could be due to many possible reasons.
First would be to ensure you have good clean oil and at the correct level. ATF or Dex ll or lll is the recomended. Oil should be changed at 300hrs max and 100hrs if used a lot. But if you have had a severe load like had a rope tangeld, then the oil should be imediatly changed.
The fill level on these boxes are measured with the stick just inserted, NOT screwed down. It is important the correct level is maintained.

[avazquez]

As always, great advice....

thanks

[Tom Spohn]

Folding prop.
I had a very worn folding prop driven by a Hurth trans. It "clunked" when engaging forward or reverse. Changing the prop took away the noise.