PRM 160

[RLWP]

Would you like to know what's inside your PRM 160 gearbox? I have just finished rebuilding one, so I thought I'd share some pictures. This box is similar to some of the other boxes, like the 260

 

The basic box has three assemblies that do all the work, the input shaft, the layshaft and the output shaft.

 

Here are the input and layshaft lying in the gearbox cover:

 

 

On the right is the input shaft, on the left the layshaft. In this view, the cover is upside down. The splines at the front of the input shaft engage with the driveplate. On the back of the layshaft is a sticky-up casting. This is the oil pump. These two shafts are permanently coupled together by the large gears at the far end, so one goes the same way around as the engine way, the other the opposite way. This is how this box gets it's reverse direction.

 

 

 

Here is the output shaft engaging with the input and layshafts - remember, this is upside down in this picture. The output shaft sits in the bottom of the box. Again, the output shaft is permanently engaged with the drive pinions on the input and lay shafts

 

How does it work then? Each shaft has a multi-plate clutch on it:

 

 

There are a stack of plates driven by the shaft, and a stack of disks splined onto the pinions. Each shaft has a hydraulic piston that squeezes this clutch to engage the drive.

 

This is the reverse pinion with a drive disk - this one is a bit tired

 

 

And here is one of the driven plates:

 

 

The plates are driven around by six of these tubes, which are bolted to the big gear on each shaft

 

This is the piston sticking out of the big gearwheel:

 

 

It is this piston that squeezes the clutch plate stack and engages the drive. For comparison, here is the piston when drive is disengaged:

 

 

The shaft rotates inside the casting on the left, which carries the oil to and from the piston.

 

So, moving the lever on the valve block pushes oil into one of these shaft assemblies and drives one of the pistons out. The piston squashes the clutch plate stack and locks the shaft to the small pinion. This pinion drives the gearwheel on the output shaft, and Bob's your uncle

 

Richard

Edited May 13, 2012 by RLWP

[cereal tiller]

Would you like to know what's inside your PRM 160 gearbox? I have just finished rebuilding one, so I thought I'd share some pictures. This box is similar to some of the other boxes, like the 260

 

The basic box has three assemblies that do all the work, the input shaft, the layshaft and the output shaft.

 

Here are the input and layshaft lying in the gearbox cover:

 

 

On the right is the input shaft, on the left the layshaft. In this view, the cover is upside down. The splines at the front of the input shaft engage with the driveplate. On the back of the layshaft is a sticky-up casting. This is the oil pump. These two shafts are permanently coupled together by the large gears at the far end, so one goes the same way around as the engine way, the other the opposite way. This is how this box gets it's reverse direction.

Thank you ,richard,very informative pictures and text.

 

reminds me of late night working to repair dead tmp's and borg warner reversing gears,the P.R.M. was the most robust of all of them,probably still is?

 

 

Here is the output shaft engaging with the input and layshafts - remember, this is upside down in this picture. The output shaft sits in the bottom of the box. Again, the output shaft is permanently engaged with the drive pinions on the input and lay shafts

 

How does it work then? Each shaft has a multi-plate clutch on it:

 

 

There are a stack of plates driven by the shaft, and a stack of disks splined onto the pinions. Each shaft has a hydraulic piston that squeezes this clutch to engage the drive.

 

This is the reverse pinion with a drive disk - this one is a bit tired

 

 

And here is one of the driven plates:

 

 

The plates are driven around by six of these tubes, which are bolted to the big gear on each shaft

 

This is the piston sticking out of the big gearwheel:

 

 

It is this piston that squeezes the clutch plate stack and engages the drive. For comparison, here is the piston when drive is disengaged:

 

 

The shaft rotates inside the casting on the left, which carries the oil to and from the piston.

 

So, moving the lever on the valve block pushes oil into one of these shaft assemblies and drives one of the pistons out. The piston squashes the clutch plate stack and locks the shaft to the small pinion. This pinion drives the gearwheel on the output shaft, and Bob's your uncle

 

Richard

[twinpot]

I shall look at my gearbox with utter admiration from now on,thanks for posting that.

Regards.

Twinpot.

[RLWP]

It's a pleasure.

 

It seems that the clutch plates have changed material. Here is a selection:

 

 

Top left is a new plate from a current kit. It seems to be a sintered material with a steel backing. Top right is an old one in reasonable condition. As it still has a discernible pattern, this one has gone back into the reverse pack - there isn't enough money in this job to replace both packs

 

The bottom pair are from the forward clutch. The left one is, shall we say past it's best? The right hand one has no friction material at all!

 

Naturally, the forward clutch pack was far more worn than the reverse pack

 

Richard

[granddad]

thanks for that it was very informative. My prm160 has an allen key in a housing to lock it in gear when the clutch is worn where's that and how do I do it. My gearbox is 13 years old should I replace it? when will I know its on the way out?

ken

[MoominPapa]

Thanks for taking the time to post this, Richard.

 

Naturally, the forward clutch pack was far more worn than the reverse pack

 

Really? I'd expect that the friction material would wear only when slipping during engagement. I think I probably select reverse and forward about equally often.

 

MP.

[RLWP]

I don't know about the Allen key, this one has one in the valve block which might block off a port and keep a piston pushed out

 

Our 260 has a moveable fork under the top cover which you can remove and reposition to lock the box mechanically - an essential feature for a marine box, not so important on the cut

 

And if it helps, this box must be at least 20 years old and I doubt that it has been touched since it left the factory. Even though some of the input shaft clutches were well past their best, apparently it didn't slip. It has been taken apart to rectify damage from being sunk and then dried out

 

If your box doesn't slip, I wouldn't worry about it

 

Richard

 

Thanks for taking the time to post this, Richard.

 

 

 

Really? I'd expect that the friction material would wear only when slipping during engagement. I think I probably select reverse and forward about equally often.

 

MP.

 

We do quite a bit of knocking in and out of forward

 

Richard

 

Apart from the valve block, the main difference I can see between this and the later boxes is that this one has taper roller bearings:

 

 

This view shows the short tubes that connect the casting on the shaft to the gearbox housing. The tubes are shown sticking up on the left, and engaged with the gearbox lid on the right. In the front right are the two holes that the input shaft tubes will go into

 

Richard

 

The main disadvantage of a taper roller bearing is having to mess about resetting the endfloat

 

 

There are shims on each of the shafts to do this, so some careful measuring and checking is needed

 

Richard

Edited May 13, 2012 by RLWP

[Timleech]

 

Apart from the valve block, the main difference I can see between this and the later boxes is that this one has taper roller bearings:

 

 

 

Richard

 

I think one big difference between the 160 & 260 is that the 160 uses Unified threads whereas the 260 is metric. Maybe metric dimensions generally, not too sure on that.

 

Tim

[casper ghost]

The PRM 140s I have came from a boat that had taken on water. I took the box apart, cleaned everything, replaced all the seals. Looked at all the parts with no real idea what most of them did, but all looked ok, so I put it back together and it's worked fine since.

My engine rotates the wrong way so I run my gearbox in reverse when going forward, maybe that will help.

 

 

Casp'

[RLWP]

I think one big difference between the 160 & 260 is that the 160 uses Unified threads whereas the 260 is metric. Maybe metric dimensions generally, not too sure on that.

 

Tim

 

That is quite possible, this seems to be an imperial box.

 

This is the output shaft in it's housing:

 

 

On this box, the bearings are all a push fit. From my manual, it seems that the later boxes use a bolt to retain the front bearing and gear. With this box, you have to push the shaft out of the bearing by pushing the gear up against the big web in the casting

 

Here is the front bearing of the output shaft being installed:

 

 

Richard

Edited May 13, 2012 by RLWP

[simon&jan]

Nice one thanks Richard, nice to see wot lurks under casings , any internal oil filters in there ?

[RLWP]

Nice one thanks Richard, nice to see wot lurks under casings , any internal oil filters in there ?

 

None at all. From the discussion on Chris Pinks twin engined boat, it seems that PRM originally fitted an external filter, but discontinued it.

 

There is a magnet on the drain plug...

 

Richard

[casper ghost]

None at all. From the discussion on Chris Pinks twin engined boat, it seems that PRM originally fitted an external filter, but discontinued it.

 

There is a magnet on the drain plug...

 

Richard

The 140 originally had an oil filter but when I contacted PRM for a new one they said they only sell a blanking plate now as they no longer feel a filter is needed.

Casp'

[RLWP]

A few pictures to complete the story

 

This is where the valve block bolts onto the top cover. You can see the traces of the oil passageways on the machined face. This also shows the two pairs of tubes to the two pistons

 

 

This is the front of the gearbox:

 

 

Note that all of the mounting holes are in the bottom half of the box. It is possible to remove the top of the box and some of the internals without taking the gearbox out of the boat but...

 

 

The ends of the shafts have push in plugs to seal them off. The design of the box is very clever, particularly the way the joins in the box and the way that oil is moved around. It uses late 20th century technology extensively, like O-rings

 

Unfortunately, this can also cause a problem. If you take the top cover off to work on the box, and replace the O-rings on these plugs, then it can be very difficult to get the top cover to sit down properly again. Been there, done that, cried into my tee-shirt.

 

Hydraulic pressure comes from a gear pump driven in this gearbox by the layshaft:

 

 

This one was in pretty good condition, so I packed it with vaseline, sealed it with Loctite and refitted it.

 

Richard

 

The 140 originally had an oil filter but when I contacted PRM for a new one they said they only sell a blanking plate now as they no longer feel a filter is needed.

Casp'

 

That's also what Tim said in the other thread. Judging by the tiny amount of crap in the bottom of this box, that's probably correct. Even after this box had been swimming...

 

Richard

Edited May 13, 2012 by RLWP

[Keeping Up]

Brilliant posting Richard, I'd wondered what they looked like inside.

 

Back when I had a Triumph Vitesse I bought a Haynes manual, and turned to the chapter about gearboxes, There was a diagram labelled "the gearbox exploded" and I remember wondering "How did they know what has just happened to my gearbox"

[RLWP]

Brilliant posting Richard, I'd wondered what they looked like inside.

 

Back when I had a Triumph Vitesse I bought a Haynes manual, and turned to the chapter about gearboxes, There was a diagram labelled "the gearbox exploded" and I remember wondering "How did they know what has just happened to my gearbox"

 

Easy, it happened to every Triumph gearbox eventually

 

This is the other end of the gearbox with eh flange nut being tightened:

 

 

I'm not left-handed, I can't operate the camera with my left hand...

 

I want to check the oil pressure from the pump, and that it goes into gear both ways, then it's back into the boat for this box

 

 

Richard

Edited May 13, 2012 by RLWP

[BlueStringPudding]

Thanks, Richard. Really interesting and reminds me what my poor gearbox is trying to achieve when the oil has dribbled out the leaking drainage plug at the bottom, and I've forgotten to top it up!

[luctor et emergo]

I've reported this obviously pornegraphic thread to the moderators...

 

 

Filthy....

[RLWP]

I've reported this obviously pornegraphic thread to the moderators...

 

 

Filthy....

 

Psst, wanna see the inside of a Gleniffer gearbox?

 

Richard

Edited May 13, 2012 by RLWP

[luctor et emergo]

Psst, wanna see the inside of a Gleniffer gearbox?

 

Richard

I say you cad, what do you think...

 

 

Oh, go on then.......

[Grace and Favour]

Richard,

 

Many thanks for the RLWP guide to PRM boxes.

 

Informative, interesting and entertaining

[Victor Vectis]

Yes, interesting and good pics too.

 

Is it Tawny Owl's gearbox undergoing the surgery? All part of the plan to have her in tip top condition for the BCN Challenge perhaps.

 

I have a phobia about gearboxes. It stems from my cack handed reassembly of a BSA Bantam engine in 1976. I could only engage 2nd and top gear but being a callow yoof and inexperienced in these things didn't realise this until a journey from Newcastle to the Eden Valley, through Alston and over Hartside. I was wondering why I had to get off and push the bike round the hairpins

 

SAM

Ryde

IOW

[RLWP]

Hi Sam, no this isn't Tawny's gearbox. This is a new venture for me, and one I find I enjoy very much

 

Richard

[Keeping Up]

A friend of mine had a giant motor-scooter called a "Maico Mobil". It had a 4-speed gearbox but after he'd rebuilt it, it had an extra neutral plus two forward gears and one reverse gear!

 

It was great fun trying to ride a scooter the size of a two-wheeled milk-float, in reverse

[Victor Vectis]

A friend of mine had a giant motor-scooter called a "Maico Mobil". It had a 4-speed gearbox but after he'd rebuilt it, it had an extra neutral plus two forward gears and one reverse gear!

 

It was great fun trying to ride a scooter the size of a two-wheeled milk-float, in reverse

 

The other bike I had at that time was a James Captain (an excellent example of why Japanese bikes came to sell so well ) with a 199cc AMC 2-stroke engine. I don't think I had the timing quite right as occasionally when the clutch was pulled in the engine would give an extra noise and start to run backwards. It was always a bit disconcerting to try and pull away from traffic lights only to find yourself going backwards!