Velvet Drive problems

Apologies for the long post but I wanted to explain fully what I have done and what the problem is straight away.

I can’t work out why my clutch pack has failed? I have a Velvet Drive AS1-70C gearbox with a 2.1:1 reduction ratio it is coupled to a small diesel engine (54hp) and we run the engine at about 1000 - 1200 rpm to give us cruising speed (canal boat doing 3-4mph). We did a three hour run with no problems and moored the boat for the night, the next day I started the engine and there was a horrible grinding coming from the flywheel area, when I engaged forward we had little to no power same with reverse. I checked the oil and it was nice and pink but a little low, I topped it up but that did nothing. I removed the gearbox and saw the drive plate was broken but it looked like it had been for some time? When I opened the gearbox I instantly saw the reverse clutch had snapped in two and the internal teeth looked mashed! So I ordered a rebuild kit, a new oil pump (thought it was best to replace) and a new drive plate. The info in the 70C service manual is a little sketchy but I am no novice mechanically, so armed with my digital camera I stripped the drive and replaced all the seals, clutch plates and gaskets all went well. The only problem I came across was I could not figure out exactly how the forward clutch system worked? Looking at it in my opinion no matter how much the forward piston pushes the clutch spring it can’t put pressure on the plates because the snap ring bottoms out on the gear teeth of the ring gear? I called a place in the US and queried the snap ring that the clutch spring sits on, he said it looks weird but that’s how it works all he said was to ensure I put the thicker of the two springs in first which I did do. When I assembled the clutch spring compressed the top of the unit (forward piston) into the ring gear and inserted the last snap ring there was obviously pressure pushing down on the snap ring in the ring gear but you could easily move the clutch plates I would say there was about 1mm or more of play. In the 70C there is no rear snap ring and no mention of having to set any clearance with gauges. Totally baffled on how the clutch would bite I rebuilt the box, all looked well on the bench. I re fitted the gearbox back in the boat and got drive, I took the boat out for an hour to test the box and that’s when things got weird. Firstly moving forward 2mph all was well however put more power on and we slowed down, put the box in reverse (700rpm) and there was this loud constant clunking noise increase the revs (1200rpm) and clunking went, boat worked well in reverse apart from clunking. I checked the engine rpm with the gearbox output rpm and in forward at 700rpm I only got 280rpm out, then at 1000rpm I got 280, then at 2000rpm I got 280 out. Obviously for some reason the clutches were slipping? In reverse at 700rpm I got 330 out and at 1000rpm I got 480 out so apart from clunking all looked well there. I need to find out if I was given the correct ATF as it was supplied by the place I got the parts from but they re bottled it so I have no knowledge that it was totally correct but they do state it is for use in 70C, 71C, 72C borg gearboxes. I was careful checking the oil level and I know it was spot on so I was stumped. Today I decided to change the oil with some that I knew was Dexron II to see if the oil was the problem, I sucked the oil out and instantly saw bronze dust and discolouration as I got near the bottom of the fluid so I know the clutches have been slipping bad! With little to loose I started the engine to get the pump to shove as much oil back into the sump as I could so I could get more of the oil out, putting the box in fwd and reverse I noticed a strange thing, we obviously had no forward output but I did have reverse and it still looked good but the clunk had totally gone? Obviously I only ran the engine in total for about 10 seconds and moved through forward / reverse to get oil out. It worked and I took out another 0.5 litre. I re filled with new oil Dexron II and fired it up exactly the same problems as before? I am at the point where I will get a pressure gauge and read the pressures on the box to see what valuable info that gives me, then I will remove the box and replace the clutch plates again. Hopefully someone with greater knowledge can clear some of my problems up and clarify how the forward clutches work? What I don’t want to do is rebuild again and have exactly the same problem!
Thanks in advance.

 

Reading the pressure at the top test point is the first step. Put it in gear and see what happens. If it drops there is a leak in the sytem somewhere. The clonking noise was probably forward engaging at the same time as reverse. If the shifter valve is damaged or worn, it can send oil pressure to both forward and reverse.

 

Hi Gonzo

Thanks for the info, I assume testing the pressure will show if the shifter valve is to blame, the odd thing is the box worked fine then failed suddenly? Do you have any idea how the forward clutches bite with all that play? Form having a look around the clutch spring is to push the piston back so that would account for one thing, but I still don't see how the piston moving out makes the plates bite?

Do you know of any way I can give the box a pre-test on the bench?

Thanks,

 

The piston compresses the clutches and the plates. You can bench test it if you connect a motor of some kind to make it turn at least 500RPM. However, for a proper test you need some kind of load and to turn it faster. Where in the UK are you?

 

Thanks for the reply, I have just got the pressure gauge and couplings if I get chance I will have a go tomorrow.... I am miles away from you unless you want a holiday in Blackpool! The problem I have with the clutches is I can easily see how the reverse clutch bites by the piston pushing on it but as far as I can see I dont see any way where the forward piston can put any force on the forward clutch plates?

 

I got to test the pressure today, I don’t think it clears much up?

From the line port:-
It read 135psi in neutral, 135psi in forward and 135psi in reverse.

From the reverse port:-
It read 0psi in neutral, 0psi in forward and 135psi in reverse.

The max pressure I could get it to hit was about 170psi but then it would start to fluctuate so I guess that would be the pressure valve opening?

Any help would be greatly appreciated. The company that supplied the questionable ATF fluid also came back and said it was Dexron II but I am still not sure I doubt they would admit if it was wrong??

 

135 psi should be fine. That means the pump and seals are OK.

 

Hi Gonzo

Well I got the box sorted, there is a guy called Eric on another forum who knows these boxes inside and out! It turned out the clutch pack was stacked correctly but in backwards, now I have loads of power.

Do you know if you are allowed to post a link to another forum? There may be some useful info there for others?

Thanks for the help Gonzo.