PBO reader Charles Beddingfield conducted a step-by-step survey to diagnose the noise in the 40-year-old PRM 100 gearbox on his ship

As a shipowner who is practical enough to handle most maintenance, I still have to think twice before delving into the internal structure of the gearbox. People imagine the need for special tools and workshop facilities, perhaps a hydraulic press, and the prospect of shrapnel bounce off the wall as some unexpected springs escape.

However, after 15 years of modification, when the Tudor Rose was relaunched in 2018 (what can I say? I work very slowly!) I found myself pricking my ears to her old gearbox, which was made by Coventry. PRM manufacturing.

It was second-hand with her BMC 1.8 diesel engine a long time ago, and now its gears have been used for at least 40 years. Except for the slight hum, it works very well. No one seems to have noticed, but as the gearbox travels backwards quieter, when the load has more gears than the front, this strange behavior lingers in my mind. Suspecting that the thrust bearing is malfunctioning, I decided to check its internal structure.

Tudor Rose's gearbox is PRM 100 or 140-the ID plate has long since disappeared, so it is difficult to distinguish. Either way, it is an old model that is no longer manufactured. PRM-Newage does not provide spare parts, but some parts are commercially available. If you are desperate, you can always look for a retired donation box. They are very popular on the Broads charter fleet and on the canal. I bet there is a lot of accumulated dirt there. They have long been forgotten under the workbench of a certain ship repairman, but they are too good to be sent. scrapped.

These procedures are applicable to 100, 140, 175, 250 and 265 models, which are basically similar in structure from the drawing. Later models used different bearings and required tricky adjustments with spacers. You can get the PDF of the maintenance manual for free from the PRM-Newage website.

Before starting, drain the oil and clean the outside of the tank thoroughly to prevent old paint and dirt from entering the bearings and hydraulic system.

When prying open the main casing, be careful not to damage the mating surface without gaskets. When removing the auxiliary parts, if you want to reuse them, please be careful not to damage their O-ring seals. The only common gasket is located under the top access cover and hydraulic control block.

No special tools are required. Although I found a pair of angled circlip pliers useful, they are not required.

1. Loosen and remove the top access panel, which is equipped with the emergency take-home equipment selection panel.

2. Loosen and remove the oil pump. Mark the oil pump and housing so you know to put it back in place.

3. Loosen and remove the input shaft end cap. Be careful of those O-rings.

4. Now you can release the upper part of the housing. Integrate the top with the hydraulic control block.

5. Remove the cover from the casing, and the input shaft and counter shaft can be taken out completely.

6. The output shaft is now displayed in the main housing. This is more difficult to remove.

Disassembling the shell, the whole work is exposed. After 40 years of service, the internal structure of the gearbox looks like new, which is undoubtedly due to the excellent lubrication inherent in the manufacturing and working methods of the gearbox.

In this type of hydraulically operated gearbox, the input shaft is rotated by the engine and drives the countershaft through a pair of large gears.

Riding freely on the input shaft is a smaller pinion, which is connected to a multi-plate clutch-the countershaft has the same.

Therefore, whenever the engine is running, the input shaft and the counter shaft rotate in opposite directions, and the smaller pinion gear meshes with a larger gear on the lower output shaft, but remains stationary as long as the shift lever is in neutral.

The role of the clutch is to lock the pinions to their respective shafts, so you can move forward or backward, depending on the clutch activated by the lever.

On this gearbox, the control rod directs pressure oil (from the oil pump driven by the countershaft) to one or the other feeder on the shaft.

The feeder activates the piston, forcing the clutch to engage. So when forward is selected the input shaft clutch engages, and for reverse the layshaft clutch is engaged.

Therefore, all gears are permanently in the mesh, and when the front or check valve is selected to drive, there is no grinding, bumping or bastard.

1. The workshop manual indicates that the large snap ring near the large rear bearing should be removed, and the entire shaft including the bearing should be driven forward as far as possible (may be about one-eighth of an inch) to remove the sealing cover from the shaft. Front. It would be great if you have a circlip pliers large enough and at the right angle for this men's size circlip. Mine is not up to the job, so I designed an alternative program.

2. First, I loosen the output half coupling nut. This is very tight, so when you unscrew the nut, you need to design a strong rod on a few bolts on the flange, so...

3. …then the half coupling easily slips off the shaft spline.

4. The oil seal housing can now be loosened and removed.

5. This allows the shaft to be driven forward with a hammer as specified, without the large bearing and snap ring being disturbed. Use a piece of wood to protect the shaft from hammering.

6. This will eject the aforementioned sealing cap from the front of the shaft. Likewise, the lid is sealed by an O-ring, so be careful not to damage it when you pry open the lid.

7. Below the cover is the fixing screw at the end of the shaft. The prize opens its tab washer and unscrews it to remove the fixed washer below.

8. Now turn the gearbox upside down and support the housing with a block so that you can carefully place the sledgehammer on the front end of the shaft so that the shaft and the large rear thrust bearing are removed from the housing downwards (ie backwards), leaving behind The annoying snap ring is in place. Hammering seems cruel, when the large output gear is against the inner web of the housing, the hammering must be done carefully. The key is the weight of the hammer, not the heavy blow. Use the socket as a drift.

9. When the shaft is driven, it leaves the gears and front roller bearings, which can now be taken out of the housing, and the shaft itself can also be removed from the end.

You can now inspect the worn bit and decide what (if any) needs to be updated.

Custom parts such as gears are no longer available, and serious failures will cause the gearbox to be scrapped. But bearings, oil seals and O-rings can be obtained from simplybearings.co.uk or bearingboys.co.uk.

The number is engraved on the inner and outer rings of the bearing. If the number is no longer listed, please search for a suitable alternative.

The seals are sorted by the diameter of the shaft and the bearing seat, the thickness of the seal, and whether it has one or two lips.

The four-point contact bearing of my gearbox is in surprisingly good condition. So where does the noise come from?

So here is the goal of my exploration. The inner ring of this kind of bearing is divided into two parts, which are designed to withstand the thrust of the drive shaft and are called four-point contact bearings.

I expected to see wear on the main output thrust bearing, but as you can see, the balls look perfect, the raceways are clean and shiny, with no streaks, grooves, wear or cracks. After assembly, the bearings rotated smoothly and silently, and I couldn't find enough failures to justify a replacement cost of around 140 pounds.

The bearings on my input shaft and countershaft are also in good condition, and since the gearbox works well, there is no need to interfere with the clutch assembly.

The front roller bearing of the output shaft was the only fault I could find. The bearing will of course not collapse or jam or anything serious enough to render the gearbox inoperable, but since the "box" has broken into pieces, I chose to replace the bearing at a cost of about £35.

PRM told me that the bearing has been gone for many years (at least from them), so I chose an alternative from simple bearing dimensions and characteristics.

New and old bearings: The old bearing RHP MRJA 20 is on the left, and its replacement SKF NJ 304 ETVP is on the right.

The old bearing RHP MRJA 20 is on the left, and its replacement SKF NJ 304 ETVP is on the right. They are all cylindrical roller bearings, but the difference is that the roller ring of the old bearing has a shoulder on the inner ring only on the outer ring, while the new bearing has rollers on the outer ring and only one on the inner ring. Shoulders.

In either case, the bearing should be installed so that the shoulder and the roller together prevent the outer ring from being driven too far. Holding everything in your hands will make this clearer.

Reassembly is mainly the reverse process of disassembly, but the following precautions may be helpful:

1. Assemble the thrust bearing, half coupling, oil seal and its shell on the output shaft and bite the coupling nut (it can be fully tightened later). Insert the shaft into the housing and slide it on the washer and gear while walking. The inner chamfer of the gasket must face the shoulder of the shaft.

2. Turn the case over and fully drive the shaft by tapping the coupling until the thrust bearing rests on the large retaining ring. Now you can fix the oil seal housing with bolts, taking care not to damage its O-ring. Applying a layer of oil will help it squeeze into place.

3. Turn the whole stack over and place it on the flange of the coupling half. Place the inner spacer washer on the shaft, then the inner ring of the front bearing and drive down, using a convenient size sleeve as a punch until they rest on the gear. Then insert the outer ring of the front bearing and push it in until it is properly seated. For this, I used a piece of wood and an old wheel bearing race, slightly smaller, as a drift. If you arrange the bearings in the right way, you can't really drive them too far, but there is no need to press them down so much that you fix the rollers on the shoulders of the track. Now replace the bearing fixing washers, lug washers, and fixing screws, and then rebend the lugs to lock them in place.

4. Insert the front end cap (note the O-ring) and tap it to be flush with the housing. Check whether the shaft rotates smoothly, without obvious end floating, radial slack or grating.

Readjust the two halves of the gearbox

The instruction says to assemble the output shaft into the housing as above, then place the input shaft and counter shaft in place, and finally put the housing and bolts on.

However, since the oil feeder sleeve must be aligned with the channel in the control block, when working alone, I found it easier to invert the upper shell and put the input shaft and secondary shaft on it, then invert the main shell and lower it into place.

This is a heavy work, so I temporarily made a handle fixed by a drain plug.

If you turn it upside down like this, don't forget that the input and counter axis will appear to be rotated from left to right. If you get them wrong, it won't cause any harm, but you may only notice when you lift your weight back to the boat and find that you can't fix it to the engine again.

Clean the mating surface of the housing carefully beforehand and apply a thin layer of cement (Halfords provides an option) because there is no gasket at the joint.

Carefully align the end faces of the two halves, because there are no pins to ensure perfect alignment, and the individual studs are not that accurate.

Finally, fix it on the rear end cover of the input shaft and the oil pump with bolts.

When replacing the top access panel, you can cut enough new gaskets from the thin card as needed-the side of the cereal box is ideal.

Make sure to install the access panel so that the emergency drive selector board secures the emergency drive dog ring forward to disengage it (see the first photo in the step-by-step section).

The retaining ring may be located on the input shaft or the countershaft, depending on the direction of rotation of the engine and propeller.

Finally, a touch of fresh paint makes this old machine look decent.

Strictly speaking, the gasket gasket mentioned above should be updated. It is also a good idea to replace the old hydraulic hose connected to the oil cooler. My mine was specially assembled in about 10 minutes, and the cost of each local hydraulic company was 22 pounds.

Sharp-eyed readers will also find that my gearbox lacks an oil filter. There is no doubt that a suitable filter can be obtained somewhere, but PRM confirmed that since the oil filter is omitted in later models, the filter port can be simply closed, which will only send the oil back to the oil pan.

Having said that, don't forget to top up again before trying. Although PRM 100 and 140 are described as hydraulically operated gearboxes, they use ordinary 15w40 engine oil (mineral oil, not synthetic oil). Do not use hydraulic oil, it will paralyze the box, but please note that there are some mechanically operated PRM models that do use hydraulic oil. Check the operation manual.

Finally, in my case, what is the use of all these tasks? Although I did not find a serious failure, only replaced a bearing, but I have a lot of insight and new confidence in my gearbox. Midway through the season after the (minor) refurbishment, it provided excellent service. The previous noise is now even unnoticeable to me, which either means that the new bearing is reasonable, or my hearing is degraded faster than I thought Be fast.

Anyway, after seeing what is inside my gearbox and how strong it is, I can be sure that this thing will not collapse in a pile of worn-out parts.

I also learned how to use a mechanical drive dog that goes home in an emergency. In the case of terrible hydraulic pressure or clutch failure, PRM is fortunate to use it.

All in all, I am very happy to finish this work tirelessly.

Originally published on PBO Dec19

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