PRM Delta

The PRM Newage 160 is a hydraulic, constant mesh gearbox with two plate clutches.

Building up the gearbox, we start with the input shaft:

This shaft has the input splines on the right and the reverse gear drive gear on the left

The drive gear contains a concentric piston (the big ring thing):

Onto this shaft goes the forward gear clutch. The PRM clutch kit is very comprehensive, having all of the wearing parts and fixings:

The clutch pack starts with a driven plate:

The tubes are carried on the fixing bolts and push the driveplates around. The springs are to disengage the clutch

Alternating with the driveplates are the driven plates (!):

The teeth in the middle of these engage with a spline on the output gear.

This is the whole assembly with the output gear installed:

Here we have the three shafts in the rear half of the casing:

Top right is the input shaft, top left is the reverse shaft permanently meshed with the input shaft. At the bottom is the output shaft.

Here we have the output gear installed:

The output shaft gear meshes with the two output gears, one on the input/forward shaft and one on the reverse shaft.

On the end of the input shaft is a gear pump:

The top image shows the gears in their housing, the bottom one shows the feed and delivery pipes. Oil is drawn in from one of the holes, travels around the outside of the gears and exits through the other hole.

It is this oil that makes the gearbox work. Oil is pumped into the space behind the piston on one of the gear/clutch clusters, squeezing the clutch plates together and locking the output gear to the shaft

This box turned up with a few problems, some of which may be connected. It had been working with an R&D driveplate that had worn out http://www.primrose-engineering.co.uk/rd-driveplate/

The out of balance load had wrecked the input bearing:

Now, it could be a coincidence, I would be wary of buying bearings from this source:

These are the bearings:

If you are confused, that’s because these were once cylindrical rollers. They have got jammed, spun, and worn away until some of them are now nearly spheres! This was replaced with a nice, quality, SKF roller bearing – much better

The next issue was a bit more unpleasant. Inspecting the front casing I found this:

Notice the crack running across the casting. From the outside, it looks like this:

This partially explains what has happened. Either out of balance forces from the worn driveplate, or some of those rollers tumbling in the broken bearing, or just the forces between the gears – more likely all three – have broken the casing. This was made easier to do by the two stud holes lying along the high stress line. The hole between the bearings is a particular nightmare. Both these holes are moved on the later PRM 150 box

A new casing would mean scrapping the box, as would welding up and remachining the casting.

Looking at the fixings on this casing, all of them lie above the crack. As a simple fix, I drilled and tapped two new fixing holes below the crack:

Here are the new fixings in the gearbox backplate:

This box is now installed in it’s boat and is working fine. This was a pragmatic fix for this box, doing what was needed and no more

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