Gearbox worries me

[Sid Charles]

Yes me again

There is something about my gearbox which concerns me, and it appears to have only recently materialised. When I put it into forward all is well, but when I go into reverse it is really aggressive and goes in with a right clunk/thump, all the fluids are good and the coupling is good, no backlash anywhere, what could cause the smash into gear feeling?

[mrsmelly]

Looks nice and clean in there 

[BoatingLifeUpNorth2]

@Tony Brooks your wanted again 👍

[b00ke23]

It would be good to know what sort of gearbox it is. It looks a bit like a PRM mechanical gearbox but can't be sure. There should be a plate on it with the model number. 

[Sid Charles]

I think it is a PRM but I'm sure the make won't help find the problem

[Tony Brooks]

36 minutes ago, BoatingLifeUpNorth2 said:

@Tony Brooks your wanted again 👍

 

Edited to remove misinformation - PRM make no mention of any "servo" mechanism to help enauge the clutches, in this case cone clutches.

 

Not sure if I can help with this one.

 

If we assume that ahead is working as it should then I can't really think of much that would allow reverse to go in with a thud of clunk.

 

Normally the astern mechanisms tend to have an easier life than ahead. A rather large generalisation and may well ignore the inland practice of emergency astern and rev up to stop the boat in an emergency, but it seems ahead is more likely to slip through wear before astern.

 

Now, mechanical boxes tend to be far more clunky than hydraulic ones, so I wonder if reverse is more or less normal, but ahead is slipping a bit through plate wear.

 

@Sid Charles  Take the control cable off the lever on the gearbox and try operating the lever directly, you may need a length of pipe or some such to give you a bit of leverage. If both react is the same way, then it probably indicates the Morse control cables need adjusting.  If it still acts as you describe, I fear the box may need stripping to see if anything is slipping or is worn. I assume that you are using ATF fluid, typically called Dexron in the box. Dexron followed by any number will do, so choose the cheapest. If you have engine or gear oil in it, then change it for Dexron and see if it improves.

Edited February 21 by Tony Brooks

[b00ke23]

25 minutes ago, Sid Charles said:

I think it is a PRM but I'm sure the make won't help find the problem

 

Are you serious? The make will certainly help....for a start we can establish if it's a hydraulic or mechanical gearbox which is instrumental in suggesting potential fault finding procedures. 

Edited February 21 by b00ke23

[Tony Brooks]

26 minutes ago, Sid Charles said:

I think it is a PRM but I'm sure the make won't help find the problem

 

That is where you are wrong, Hurth mechanical boxes operate differently to PRM mechanicals and also use different types of clutches. While a slipping clutch is a slipping clutch, be it cone or multi-plate, how the force is applied to the cutch plates differ

[nicknorman]

A lovely photo of the engine. Just a pity there is a great big leg obscuring the bit in question ie the gearbox! However it does say “ATF” which suggests a mechanical box

[Tony Brooks]

I was wrong again, I can now see the bolts holding the two halves of the case together, that is a fair indication it is a Hürth that uses balls running up  ramps to provide the bulk of the clamping force for the clutch plates. This still does not later what I said above about trying it on the gearbox lever, ensuring it ATF oil (Dexron) in it, and then it is possibly needing a strip down and internal inspection - or changing for a PRM mechanical box.

 

 

[Quattrodave]

3 hours ago, Sid Charles said:

I think it is a PRM but I'm sure the make won't help find the problem

 

Oh ok, so you are a gearbox specialist, best diagnose it yourself then, other than that, take your food off it, give us a part numbers, and at least give us a fighting chance...

[PaulJ]

Its same /similar box as mine. Hurth -although mine hasnt got an oil cooler on it.

Quite often I find (on a variety of boxes) that the adjustment is out on the control cables. Over adjusted on one gear and underadjusted in the opposite. So revs pick up and it thunks in..

It should go fully into gear both ways before the revs rise. 

Nice simple fix with a bit of fiddling. Fingers crossed for you 

Edited February 22 by PaulJ

[Sid Charles]

I'm going to do some tests on the cables later today, but I watched a video of someone rebuilding a Hurth box and at the end he ran it and when he put it into gear it jumped like mine does, I am now thinking my idle speed is too high, causing the jump into gear, I am having a bit of trouble getting a nice steady idle speed, I have set it when the engine has just warmed up, but I have noticed that when the engine gets much warmer, the engine speed increases. I am going to play around with it today.

[Sid Charles]

Well I went to the boat and did some checking and testing regarding the engine/gearbox, first thing was make sure the morse cables were correct, I knew they were because I put a brand new control on a few months ago (for all who are not familiar with me I have rebuilt this boat over the last 2 years and it is out of the water) I then started the engine and as I have stated, it is really lumpy from cold with diesel knock, because of this I adjusted the idle speed up. And so today I increased the engine revs until it warmed up, I then went back to the idle speed and reduced it back down, but if I reduce it too much the engine knock returns, so Let it run at it's happy speed, I then went into forward gear and it is quite smooth, then I went back to neutral and then slowly into reverse, but even though a bit less violent, it still went into reverse with a big clunk making the engine lurch a bit ( I am an Engineer and a motorcycle man all my life, so I know what is right and what is not quite right) and this seems too much of a crash/bang to be good for both the box or engine, and so I wonder if anyone has any insight into this. I didn't run the engine for too long because having replaced the thermostat last week the pipes leading to the skin tank and calorifier now warm up, but my next problem reared it's head, the expansion tank is cracked and when everything began to get hot it was spitting and hissing, so I had to stop my tests until I find a replacement expansion tank, see pics of the gearbox identity and my tank.

[Ex Brummie]

I've not seen anywhere in the posts where you specify the RPM of the engine. Running it to a sweet spot may mean you are running it quite fast, and whilst it goes forward which is a 2 or 3 to 1 ratio, when selecting reverse will be 1 to 1 which is going to alter the immediate strains on the gearbox if too fast. If you disconnect the morse cable from the gearbox selector, does it still do the same if you select gear manually?

[Tony Brooks]

OK, so you will be familiar with wet multi-plate clutches. you have a pair of small ones inside your gearbox. One for ahead and one for astern. As far as I can gather, they are both the same. One is on a gear that is free to spin on the input shaft and drives the output shaft through two gears - ahead. The other is on another slightly smaller gear that is free to spin on the input shaft but drives the output shaft via a reverse idler gear, hence the reversed direction of the output shaft.

 

The manual now gets a bit opaque, so some of this is informed conjecture.

 

The gearbox lever moves a large "selector" block forwards or backwards, this is cylindrical with a recess in the centre to accept the selector "pin" that the lever moves.

 

Either the faces of this block has grooved ramps machines on it with balls sitting in the grooves or the moving steel plate against the clutch pack does. Whichever it is the other one is a plain face.

 

When you select the gear, the block moves the balls against the plate in the clutch pack and the rotation/torque in the input shaft and the resistance of the drive line to the prop tending to keep the clutch pack from rotating causes the balls to roll up the ramps, forcing the clutch plates together. When you disengage the gear, the selector block and the clutch plate move away from each other so the clutch disengages.

 

This apples to ahead and astern, so the question is "why the difference in engagement". The only answers I can come up with is either the ahead clutch is slipping or the astern mechanism (the balls and ramps) is stiff and suddenly moves and "grabs".

 

Be aware that the input shaft end float is critical in these mechanical boxes, but I would have expected that to affect both gears equally.

 

9 minutes ago, Ex Brummie said:

I've not seen anywhere in the posts where you specify the RPM of the engine. Running it to a sweet spot may mean you are running it quite fast, and whilst it goes forward which is a 2 or 3 to 1 ratio, when selecting reverse will be 1 to 1 which is going to alter the immediate strains on the gearbox if too fast. If you disconnect the morse cable from the gearbox selector, does it still do the same if you select gear manually?

 

There will be difference in the ahead and astern ratio, but nowhere near what you quote, more like 0.3 difference. The difference is accounted for by the slightly smaller reverse gear to allow the idler gear to fit in the box.

[Wanderer Vagabond]

Just as an aside, and with no specialist knowledge on the subject whatsoever, but I noticed that you said the boat is out of the water. Are you putting it into forward and reverse drive with no water resistance on the prop? 

[Sid Charles]

28 minutes ago, Ex Brummie said:

I've not seen anywhere in the posts where you specify the RPM of the engine. Running it to a sweet spot may mean you are running it quite fast, and whilst it goes forward which is a 2 or 3 to 1 ratio, when selecting reverse will be 1 to 1 which is going to alter the immediate strains on the gearbox if too fast. If you disconnect the morse cable from the gearbox selector, does it still do the same if you select gear manually?

I don't have a rev counter and I haven't tried disconnecting the cable, but I can't see it changing anything the revs don't increase until I pull the morse lever further

[Tony Brooks]

43 minutes ago, Ex Brummie said:

I've not seen anywhere in the posts where you specify the RPM of the engine. Running it to a sweet spot may mean you are running it quite fast, and whilst it goes forward which is a 2 or 3 to 1 ratio, when selecting reverse will be 1 to 1 which is going to alter the immediate strains on the gearbox if too fast. If you disconnect the morse cable from the gearbox selector, does it still do the same if you select gear manually?

 

Here are ratios for some Hurth boxes, but the difference between ahead and astern will be similar across the range, they are a bit greater than I thought/said, but not as great as you posted

 

18 minutes ago, Sid Charles said:

I don't have a rev counter and I haven't tried disconnecting the cable, but I can't see it changing anything the revs don't increase until I pull the morse lever further

 

Well done for getting the throttle cable adjustment correct, that catches out a lot of people who have a control with a spring-loaded cable anchor for the throttle.

[blackrose]

This might be a stupid question, but if it is a mechanical box I assume you're putting it into neutral for a second or two before going into astern, not just slamming it straight from forward to astern?

[David Mack]

1 hour ago, Sid Charles said:

 

 

44 minutes ago, Tony Brooks said:

Here are ratios for some Hurth boxes, but the difference between ahead and astern will be similar across the range, they are a bit greater than I thought/said, but not as great as you posted

 

From the photo the forward gear ratio is 1.88 and the reverse ratio 2.10,which is the opposite way round from your table.

[Tony Brooks]

19 minutes ago, David Mack said:

 

From the photo the forward gear ratio is 1.88 and the reverse ratio 2.10,which is the opposite way round from your table.

 

It is not worth arguing about. I was trying to save the OP chasing what looks very like duff information to me. I agree the letters on the label suggest that but!

 

Whatever the label says whichever gear needs an idler will have the one of the gear train pair that is slightly smaller than the ahead gear, simply so its teeth clear the mating gear on the output shaft. On modern hydraulic boxes being driven "backwards" it may be that the idler can run for extended periods under load and in that case the ratios would be reversed, but mechanical gearboxes that typically do not have an oil pump for forced lubrication and rely on flooded bearings or splash it is bad practice to run them in nominal astern while driving the boat forward. I can't see any reputable manufacturer doing that, so unless we can see what the manual that came with the box says I stand by reverse having the lower ratio. In any case the difference will be nowhere near a whole number as someone claimed.

 

The PRM mechanicals are different in that they have a common 2.47:1 astern ration and varying ahead rations, only the 3.06:1 ahead option has anything like a whole number difference between ahead and astern, in that case it is a difference of 0.65. That may make a noticeable difference in engien revs sound between ahead and astern.

[Sid Charles]

12 hours ago, Wanderer Vagabond said:

Just as an aside, and with no specialist knowledge on the subject whatsoever, but I noticed that you said the boat is out of the water. Are you putting it into forward and reverse drive with no water resistance on the prop? 

This is a valuable suggestion and yes, there would be a resistance from the prop in water, that may well damp down the sudden crash

[Tony Brooks]

1 hour ago, Sid Charles said:

This is a valuable suggestion and yes, there would be a resistance from the prop in water, that may well damp down the sudden crash

I suspect the opposite because of those ramps and balls giving a servo action, the strength of which must be related to the resistance of trying to turn the output shaft (prop).

 

Another point you may not be aware of. When an engine runs, there is no scientific law that says the engine will stay still and the crank will revolve, it is only the engine mounts and any tie bar that force the block to stay more or less still and the crank to turn. In you have soft mounts, broken mounts, loose mounts, or mount rubbers that have softened then the engine will try to tip to one side each time you alter the load on it. This is called torque reaction. However, unless there is a mount fault, I think this should be similar for ahead and astern.

 

Something similar can happen in a gearbox, where the input gear tries to "walk over" the teeth of the output gear because there is a load trying to stop the output gear spinning. Again, it is the engine mounts that counter this. @Ex Brummie was correct to point out there is a difference between the ahead and astern ratio (but not the magnitude in my view) and that difference must give different torque reaction between ahead and astern as far as the gearbox is concerned, but your description suggests you have more that I would expect with the prop out of the water. It would be worth turning the shaft coupling by hand in both directions with the box in neutral, in case, for some reason, it is much harder in the reverse direction. Its a longer shot, bur check the engine mounts.

[Sid Charles]

Turning the prop shaft by hand is the same in both directions and there is no play in any direction, I think it will be OK, it's probably me being aver cautious