PRM 500 gearbox

[steve.sharratt]

Hi,

 

I know this thread is a bit old but I am currently trying to resolve my crunching issues on my PRM500. I see that a soft shift valve works but I am curious to know whether this has been used on a slower revving engine.  I have my suspicions that my Lister JP3 is idling too low (300rpm) and this is causing the crunch.  This is opposite to the high-rev scenario and I wonder whether the slow shift will work in my case. The valve block I currently have looks identical to the soft-shift version so I am not even sure if it is already fitted or the valve block just look very similar.  Currently waiting on some advise through ASAP Supplies who are talking to PRM.  Figured it would be worth asking the ‘real world’ in the meantime.

 

Cheers

 

PS. Has anyone tried the Beta Marine accumulator option.  They mention them on their website but don’t provide a parts option?

[Tony Brooks]

I don't see how a PRM 500 box can crunch. As far as I know it is a hydraulic box so the worst it should do is make a single clunk or a single jerk as the hydraulic clutches engage. Happy to be contradicted by others but I would be looking for the real reason for the crunch like a coupling moving on the shaft, clutches breaking up and so on. The gears in that box are helical constant mesh gears so can't really crunch.

[steve.sharratt]

9 minutes ago, Tony Brooks said:

I don't see how a PRM 500 box can crunch. As far as I know it is a hydraulic box so the worst it should do is make a single clunk or a single jerk as the hydraulic clutches engage. Happy to be contradicted by others but I would be looking for the real reason for the crunch like a coupling moving on the shaft, clutches breaking up and so on. The gears in that box are helical constant mesh gears so can't really crunch.

To be fair, it is more of a clunk than a crunch - either way its a big clunk and bloody annoying!  My box was recently disassembled and it is in new condition with no signs of wear so I am confident that the box itself is ok.

[Tony Brooks]

1 hour ago, steve.sharratt said:

To be fair, it is more of a clunk than a crunch - either way its a big clunk and bloody annoying!  My box was recently disassembled and it is in new condition with no signs of wear so I am confident that the box itself is ok.

 

Just my thoughts. Low revs would reduce the output volume of the hydraulic pump in the gearbox so it would tend to slow the engagement and reduce the clunk. This suggests to me as you have the clunk it is nothing to do with low revs. Therefore, I would expect the slow shift to improve matters.

[Alan de Enfield]

Could be worth checking your min/max pressures  (- it may be that your 300rpm input speed is not producing sufficient pressure to operate the gearbox correctly).

 

Two tapped holes 1/8" BSP on the top, and M18 on the side of the valve block are provided so that the pressure gauge can be fitted if required.

 

 

 

From the workshop manual :

 

 

 

 

 

 

 

Edited November 1, 2022 by Alan de Enfield

[jonesthenuke]

2 hours ago, steve.sharratt said:

Hi,

 

I know this thread is a bit old but I am currently trying to resolve my crunching issues on my PRM500. I see that a soft shift valve works but I am curious to know whether this has been used on a slower revving engine.  I have my suspicions that my Lister JP3 is idling too low (300rpm) and this is causing the crunch.  This is opposite to the high-rev scenario and I wonder whether the slow shift will work in my case. The valve block I currently have looks identical to the soft-shift version so I am not even sure if it is already fitted or the valve block just look very similar.  Currently waiting on some advise through ASAP Supplies who are talking to PRM.  Figured it would be worth asking the ‘real world’ in the meantime.

 

Cheers

 

PS. Has anyone tried the Beta Marine accumulator option.  They mention them on their website but don’t provide a parts option?

The PRM500 soft shift unit is obvious if it's fitted as it has two apple-sized black accumulators on the top of the valve block.

 

As per my previous comments this topic, our gear engagement is barely noticeable, this being on a Gardner 3LW with a tickover speed of about 420 RPM.

 

I concur with previous comments above, the gear change should not crunch, so are you sure the rest of the drive train is in good order, specifically the drive plate and all the couplings on the prop shaft? If all is good I would think hat fitting a soft shift unit will cure the issue, but is not a cheap option.

[Steve56]

Many years ago I had a similar issue on a Gardner engine. There was always a very noticeable clunk as it went into gear. To rectify I fitted the engine with a Centa Transmissions R type drive plate. This worked wonders and really smoothed out and quietened down the gear selection. I have used this plate on numerous engine installations with great success. I think the reason that the engine manufacturers don't use it as standard is as always down to costs.

[steve.sharratt]

14 hours ago, jonesthenuke said:

The PRM500 soft shift unit is obvious if it's fitted as it has two apple-sized black accumulators on the top of the valve block.

 

As per my previous comments this topic, our gear engagement is barely noticeable, this being on a Gardner 3LW with a tickover speed of about 420 RPM.

 

I concur with previous comments above, the gear change should not crunch, so are you sure the rest of the drive train is in good order, specifically the drive plate and all the couplings on the prop shaft? If all is good I would think hat fitting a soft shift unit will cure the issue, but is not a cheap option.

I have seen two basic options - one is the PRM soft shift which looks similar to the standard valve block (see ASAP Supplies options). The other is the Beta Marine accumulator (Dintra) which has the two accumulators on top that you mention.  

 

I have revised my description to Clunk, rather than crunch, based on TB’s comment.  I am confident that the system is in good order as I recently had a lot of work done on the box.  I have a midships engine so a long prop shaft and I am spinning a 25” prop so that probably isn’t helping.  Some sort of soft shift device is most likely the answer.  I am waiting on some feedback from PRM and will make a decision whether to go with them or the Beta Marine option.  Both around the £4-500 mark.  Ouch!

 

[chevron]

It’s worth talking to lancing Marine they can tell you if your controller mechanism is the soft shift one by sending them a picture of the underside. I believe that they can modify it to soft shift controller.  Mine was already a soft shift controller so bought the soft shift plate accumulators from them which delays smooths the oil flow to the clutches which 100% cured the fault. kit cost just less an £300 pounds no idea how much from beta marine. 

[steve.sharratt]

5 hours ago, Steve56 said:

Many years ago I had a similar issue on a Gardner engine. There was always a very noticeable clunk as it went into gear. To rectify I fitted the engine with a Centa Transmissions R type drive plate. This worked wonders and really smoothed out and quietened down the gear selection. I have used this plate on numerous engine installations with great success. I think the reason that the engine manufacturers don't use it as standard is as always down to costs.

That is interesting and I think that’s what the engineer who did recent work on the box was talking about.  Do you have a recommended supplier and a rough cost? 

[john.k]

Using the wrong oil will make clutches chatter as they engage ....as will worn friction linings.

[jonesthenuke]

Re your comment 

I have seen two basic options - one is the PRM soft shift which looks similar to the standard valve block (see ASAP Supplies options). The other is the Beta Marine accumulator (Dintra) which has the two accumulators on top that you mention.  

 

I may be wrong but I would think that you need the PRM valve block and the accumulators to get a soft shift.

[Steve56]

1 hour ago, steve.sharratt said:

That is interesting and I think that’s what the engineer who did recent work on the box was talking about.  Do you have a recommended supplier and a rough cost? 

The Gardner I did this mod on was the same set up. Trad style engine room long prop shaft and a big propeller. That's a lot of inertia. I've bought a couple over the years directly from Centa Transmissions Ltd. They were based in Shipley, close to Bradford. But I think now they have moved. I used to get them through a work colleague who was there marine sales engineer, but unfortunately he has now moved on to another company. I'm not really sure of the cost, but considerably more expensive than your typical R & D plate. But in my opinion so much better and well worth it.

[Tony Brooks]

1 minute ago, Steve56 said:

The Gardner I did this mod on was the same set up. Trad style engine room long prop shaft and a big propeller. That's a lot of inertia. I've bought a couple over the years directly from Centa Transmissions Ltd. They were based in Shipley, close to Bradford. But I think now they have moved. I used to get them through a work colleague who was there marine sales engineer, but unfortunately he has now moved on to another company. I'm not really sure of the cost, but considerably more expensive than your typical R & D plate. But in my opinion so much better and well worth it.

 

I in no way doubt what you say but think that a soft shift set up is probably a better engineering solution. The only way a drive plate can do what you say is by deflection/distortion of the parts that link the two halves. Even if the plate puts rubber bushes into compression as the drive takes up it will eventually stress/damage the flexible components so I think a soft shift would eliminate that possibility.

[Steve56]

This is an R type damper. You can obviously get them supplied with different options of backplate and spline to suit the application. As you can see they have rubber rollers that run up ramps. So any torque applied is gently taken up by the rollers. 

[Split Pin]

I had the same problem with a 750 fitted to a Gardner 3LW, although my tick over is 300rpm the engagement was very fierce, this is the root of the problem, when engagement either forward or reverse is selected the clutch engages rapidly without slipping, the propeller is accelerated up to speed, because there is virtually no load at this point the propeller continues to accelerate and reverses the backlash in the drive train before everything settles down hence the engagement noise. Like others I had spoken to PRM,s technical man who was very helpful suggesting that the twin accumulator modification should sort the problem, at this time the idea was very new and had been developed by a Dutch Company, another suggestion was to fit a Trolling Valve typically used in fishing boats, this allows the engine to be run at higher rpm for running ancillaries but allows the drive to be set at a  lower level, the trolling valve is a proportional hydraulic valve which in its basic form switches on and off rapidly so engaging and disengaging the clutch. A soft start valve is probably the same thing now sold under a different title. 
I didn't like the price of either of the options at that time so set of to develop my own version , After a few experiments I found that engagement was quiet if the initial pressure was reduced to 80-100 psi. So I built a valve block that integrated a 100psi relief valve and a 2 position electrically operated valve which I fitted it into the oil pump circuit, In neutral the valve is held open. So the oil pressure is restricted to 80/100psi on engagement. The valve closes after a time delay once ahead or astern is engaged when full pressure is then applied. When the gearbox oil is cold engagement is silent. When the oil gets hotter it will on occasions knock but  nowhere as heavily as without the modification. It has been on now for around 20 years without any problems. 

 

Edited November 2, 2022 by Split Pin

[Bee]

Yeah, It's annoying, mine used to clunk and I feared for keys and keyways and the box itself, tried to adjust the travel of the cable from the Morse control, never fiddled with the idle speed or speed cable as it was always properly adjusted, engine is a Beta 43 so a fairly fast idle at about 700 rpm I think.  Anyway after years of tooth rattling clunks and minute adjustments  I finally found a spot on the cable adjustment where the thing now engages with a civilised bump and I screwed the lock nut up tight. I am happy that it is fully engaged,   I don't really understand it as it should be either in gear or not in gear, the oil has been changed a couple of times and its done a lot of hours and it's all fine so perhaps there is a sweet spot. I am not going to fiddle with it to find out.

Edited November 2, 2022 by Bee