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PRM 500 gearbox


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I have a PRM 500 gearbox which clunks into gear unless you are very gentle selecting gear. The fault is well documented online and there is a soft shift controller replacement or an accumulator set up from the likes of beta. Has anyone on here carried out replacement of the controller or fitted an accumulator and this has cured the noise. Someone has said you can modify your original controller but not provided the information to do so? all threads of different forums never seem to have a positive fix. 

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8 minutes ago, chevron said:

I have a PRM 500 gearbox which clunks into gear unless you are very gentle selecting gear. The fault is well documented online and there is a soft shift controller replacement or an accumulator set up from the likes of beta. Has anyone on here carried out replacement of the controller or fitted an accumulator and this has cured the noise. Someone has said you can modify your original controller but not provided the information to do so? all threads of different forums never seem to have a positive fix. 

@Albion Worked on the problem if I remember correctly 

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Heavy clunks into gear with a hydraulic box is often simply due to the engine speed being too high at the point of engagement.  Does you engine stay in tick over when you first put it in gear or does it rev up slightly signifying that the cable needs adjusting to give it some slack?

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26 minutes ago, chevron said:

I have a PRM 500 gearbox which clunks into gear unless you are very gentle selecting gear. The fault is well documented online and there is a soft shift controller replacement or an accumulator set up from the likes of beta. Has anyone on here carried out replacement of the controller or fitted an accumulator and this has cured the noise. Someone has said you can modify your original controller but not provided the information to do so? all threads of different forums never seem to have a positive fix. 

As ditchcrawler has said I had this problem on my nb with a Gardner 4LK engine and the 500 gearbox. It nearly took my teeth out when I changed gear, never mind the teeth on the gears. It was very audible to passers by on the towpath and was embarrassing. This was way back nearly 20 years ago. As an engineer I tried various home built solutions to try and soften the rate of oil pressure rise in the box including experimenting with a length of pressure hose to allow the enclosed air to be compressed as the oil pressure rose, then adding a pressure accumulator and pressure gauge (just to monitor pressures) but nothing really worked well. Then, having read about the control system that existed for soft slip purposes I thought that a similar system to ease the very sharp oil pressure increase could work and wrote to PRM about it suggesting my idea. I got a very helpful reply from the Technical Director who agreed that my idea was a good one but saying that a Dutch dealer had just developed just such a system with a twin accumulator and modified valve block to fix the problem and offering me that conversion kit to fit myself FOC. I fitted it and it completely solved the problem. It is a known problem but fitting the modified valve block with its two accumulators will fix it. In fact, at one time, I thought that I had heard that the boxes were only supplied with that mod already fitted but I may be wrong in that thought.

Roger

PS. I still have my unsuccessful accumulator/pressure gauge modded pipe in my garage now.

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29 minutes ago, Tracy D'arth said:

Heavy clunks into gear with a hydraulic box is often simply due to the engine speed being too high at the point of engagement.  Does you engine stay in tick over when you first put it in gear or does it rev up slightly signifying that the cable needs adjusting to give it some slack?

 

Certainly adjusting but on many controls the throttle cable must be put back with the control in slow ahead or astern so when you put teh lever into neutral the sprung anchor is compressed/tensions so when you put it into gear the anchor moves and stop any tendency to rev up.

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This is all very interesting.

 

My PRM500 just goes from neutral to engaged in a blink and any slack (backlash?) from the drive means it sounds clunky.

 

PRM themselves even have a soft-shift to retro fit for these and i believe later models did have this on.

 

There is the accumulator solution available from Betamarine, but you are knocking on the door of £500...too rich for me. I found the original manufacturer but they too were looking at a fair wedge plus duties so i have left it.

 

Sorry to say no amount of gentle revs, easing, prayers or profanity works. Sometimes it just clunks.

 

Let us know what you plan to do Chevron.

Edited by Paringa
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Have in the passed recalibrated the rev counter and adjusted the idle as slow as i can go. the morse controls do not bring the engine off idle till well passed full gear selection. My boat has a 22 inch prop which I do not think helps the noise. 

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1 minute ago, chevron said:

Have in the passed recalibrated the rev counter and adjusted the idle as slow as i can go. the morse controls do not bring the engine off idle till well passed full gear selection. My boat has a 22 inch prop which I do not think helps the noise. 

That sounds to be as good as you can get.

 

Is there a lot of backlash in the transmission somewhere?

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I can find no backlash it’s a good solid set up the only thing i cannot check is the drive plate but have no chatter. the engine and box are in a beta acoustic suit case and i am often asked if the boat is electric as it’s that quiet. That’s why the box noise is so irritating. 

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I reengined our barge with a Perkins c135 and prm 500 in 2007 and must say I never had this problem and as long as the oil was changed to schedule it performed brilliantly. May be prm modified the box at some stage. It would be worth getting in touch with them for recommendations.

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7 minutes ago, Dav and Pen said:

I reengined our barge with a Perkins c135 and prm 500 in 2007 and must say I never had this problem and as long as the oil was changed to schedule it performed brilliantly. May be prm modified the box at some stage. It would be worth getting in touch with them for recommendations.

Later and new boxes seem to come with the the soft shift controller but not the spacer and accumulators 

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With that engine and box attached to a 22" prop you have a lot of rotating mass and inertia to transfer to the propshaft, what kind of flex coupling do you have presumably on a double universal jointed intermediate prop tube?

 

I have seen long shafts with UJs that jump as the gear clutch bites, that's why lorries and large cars have center cushioned bearings on the shaft.

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3 hours ago, Tracy D'arth said:

With that engine and box attached to a 22" prop you have a lot of rotating mass and inertia to transfer to the propshaft, what kind of flex coupling do you have presumably on a double universal jointed intermediate prop tube?

 

I have seen long shafts with UJs that jump as the gear clutch bites, that's why lorries and large cars have center cushioned bearings on the shaft.

My narrow boat was a Norton Canes, relatively deep drafted. It swung a large prop (may even have been 22 in but I can’t remember for sure). It had two universal joints on its fairly long prop shaft but I had it built with a large Ford Granada Guibo joint (flexible ring shaped joint in the drive shaft of the car, because I was familiar with them as I was running my own garage business) because I wanted to accommodate any drive shaft shock and the gear teeth still made a shocking noise before the soft shift was added. I also specified the highest ratio box of the two choices and therefore minimised the prop pitch so that prop walk, in reverse, was minimised (spinning faster but with a minimised pitch). Therefore the start up load with that pitch was also minimised. 
Roger

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3 hours ago, jonesthenuke said:

We have a Gardner 3LW with PRM 500 plus the soft shift unit. Gear engagement is almost unnoticeable.

 

Which soft shift do you have for my reference? The PRM one available at ASAP supplies or the accumulator version from Betamarine and others? 

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Set up from the gearbox is a very large rubber bonded joint then straight through the stern to prop shaft. Have someone who if I remove the box controller will advise if it’s already a soft shift one then decide an action plan then suspect it needs the accumulator set up.

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On 31/08/2021 at 11:57, Paringa said:

Which soft shift do you have for my reference? The PRM one available at ASAP supplies or the accumulator version from Betamarine and others? 

We have two very small black accumulators on the top of the gearbox. My understanding is that it is an original PRM part. The boat was built about 2005.

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This is a real nuisance. I used to wince whenever I put the thing into ahead, Bee is mid engined and has a long and heavy shaft and the inertia and corresponding clunk made me worry about damage to gearbox innards and keys in the shafting and stuff like that. Eventually (several years) I found an adjustment position that actually works and it just goes into gear with a near silent bump. The engine speed has always been OK. Can't remember which combination of adjustments to the Morse control and cable movement at the gearbox actually did it but I am never, ever, touching the thing unless its absolutely vital. It now feels as though the gearbox is  pulling the thing into gear instead of it just crashing it into gear.  I also had to sell my soul to the devil to get it right.

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  • 2 weeks later...

Thanks to everyone’s replies and especially Albion have fitted an accumulator soft shift had a short drive and it has transformed the gear change completely, I got advise before fitting that the original controller is a soft shift one. Unfortunately as the box used to have a trolling valve i have had to put a straight pipe instead of an 90 degree angle pipe which now hits the accumulators between the two points the trolling valve used on the controller  I need to change this pipe as it gets in the way of the sound proofing suit case over the engine trip to have one made to fit or a none trolling valve controller which does not need the pipe?

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23 hours ago, chevron said:

Thanks to everyone’s replies and especially Albion have fitted an accumulator soft shift had a short drive and it has transformed the gear change completely, I got advise before fitting that the original controller is a soft shift one. Unfortunately as the box used to have a trolling valve i have had to put a straight pipe instead of an 90 degree angle pipe which now hits the accumulators between the two points the trolling valve used on the controller  I need to change this pipe as it gets in the way of the sound proofing suit case over the engine trip to have one made to fit or a none trolling valve controller which does not need the pipe?

Excellent news and I’m glad you’re pleased with the conversion. 
Roger

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  • 1 month later...

After 10 years with a clunking PRM500, I have finally spent my money on and fitted the accumulator extra soft shift kit... transformed it. Not a clunk to be heard.

 

Thanks to others for posting their experience.

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