Gearbox oil level

[Guest 000]

Since I am soon to be the recipient of a new replacement gearbox, I want to make sure that I do everything right. This includes the oil level and its proving to be a bit problematic.

PRM Gearboxes state that the oil level must be exactly to the dipstick mark and further advise that even a small amount over will be damaging to the box. This is all very well, but, the dipstick is located at the very front of the box and my engine installation is at a 9° down angle. I set up my old box carcase in the vice and with a laser level determined that the difference in oil levels at the back end of the box between level and a 9° down angle was 17mm above the dipstick mark, and that seems rather a lot for a little box like the PRM 80. There is no info to be had on the internet about oil levels in inclined boxes, except for an online Yanmar manual which helpfully states that 'the gearbox must be level when checking the oil.'

The gearbox supplier is uncertain about where the oil level should be and PRM England don't respond to the question. I guess it must be a secret.????

[Island Time 1,245]

Chris is there a stated capacity for the GB? If so you could level it and fill it with the measured quantity, install it, then check the level on the dipstick and mark it....

With your past problem, I'd be wanting to get something in writing from the supplier, instructing what to do though...

[Guest 000]

Yes, they state the oil quantity, but there's another little problem. The dipstick itself extends for 6mm below the full mark. If I sit the box level and put in oil to the mark, and then incline the box to 9°, the oil level is then below the bottom of the dipstick which means I have no reference point when checking the oil once the box is installed. I suppose I could make a new dipstick.... why do they make it so darned complicated?

[Island Time 1,245]

Damn, you pick the good ones, right? A new dipstick in order, provided there is space below it, and it won't contact with anything important.

[wheels 543]

Yes Chris, the oil level is essential to have right. But there is some good news here. It is most important you don't over fill. Under filling is not so bad. Over filling makes them heat up something terrible and (oops, hehe, just suddenly remembered) they chew out the cone drive.
The boxes can be installed at a max of 15deg down angle, so 9 deg is well within spec. They take 0.56ltrs of Oil, so what I would do is fill with exactly that. Unless you have a visible leak, the oil is not going to go anywhere. If you have an oil cooler, any fault usually results in water getting into the box and the oil level rising. So upon checking the level, if you suddenly find the level high and/or water, you have a fault with the cooler....if one is fitted. Otherwise, when you do an oil change each year, then just drain and fill with exactly the correct amount.

[Guest 000]

Thanks Wheels, I was thinking to level the box, put in the oil up to the dipstick mark and then install it on the engine.

If its so important to have the right amount of oil, why does no manufacturer anywhere address the issue of oil level/installed inclination?

To me its pretty darn stupid to say, as Yanmar does, 'ensure the gearbox is level before checking oil.' How do they suppose that you will do that in a typical marine installation?

'Got gearbox troubles?'. 'Nah, just pulling it out so I can check the oil...'

[wheels 543]

Yeah I know. Especially when so much damage can result. And few realize that over filling boxes, even by just a tiny amount, can cause so much damage. It is not limited to only PRM if that is of any help. There would be few perfectly flat installations, if any, so you would expect a manufacturer to cover this aspect far better.
I am surprised James could not advise you on this though.
By the way, there are box designs that have the output shaft with a downward angle so as the engine/box remains level, or if the shaft angle is greater than the box is designed for.
And another by the way, if the shaft is only at 9deg, that is great. You would get very little prop walk with an angle that slight.
 

[ballystick 73]

Does being overfull, really wear out the cones?

[wheels 543]

Wow, where did you find that erice?
 

 

Does being overfull, really wear out the cones?

Yes absolutely. It is a known issue with PRM's mechanical boxes.
For any other box, even hydraulic and leaving aside any possible damage if the particular box can suffer from that, it can cause excessive pressure on seals and produce leaks and seal failure.
In engines, it can also cause seal leaks. But one other fact is that the big ends may splash in too deep oil and that can cause drag and thus excessive fuel use.

[erice 732]

sorry for not posting the link

 

just cut and paste an entire line into google search and the source should turn up

 

i've no idea if it's true on not

 

it's just the most relevant return from a search for

 

"why not over fill a gearbox"

 

which turned up a 2008 thread

 

"Why not overfilling a manual gearbox"

 

on an oil forum

 

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1563253

 

and stormrider;s reply

 

#1563286 - 08/12/09 11:38 AM Re: Why not overfilling a manual gearbox? [Re: kilou]

Too much oil in a gearbox will cause the gears to "churn" in the oil more than necessary and create heat which can be excessive and beyond the capabilities of the respective oil cooling system to handle (i.e. oil coolers, air flow cooling over the case, etc) and result in thermal runaway of the oil. In extreme cases the internals can reach temperatures that will cause distortion/melting. It can also cause to much aeration in the oil which could be more than what the stock breather system can handle, which will reduce lubrication capabilities (foamy oil doesn't lubricate well). 

Theres a fine line between effective cooling/lubrication and thermal runaway when oil system capacities are determined by the engineers.

_________________________
2009 Ford Escape V6/8400 mi/MC 5W-20
2007 Mercury Montego/50K/MC 5W-20
1976 Chevy K-10/73K/PYB 10W-40

[ballystick 73]

I see PRM acknowledge their gearboxes have a problem if overfilled, but that excerpt doesn't have much technical backing in my view. Excessive friction, heat build up, seal failure with associated venting of oil sounds feasible. I was thinking that something was forcing the friction cones constantly into contact with the hub surface, a bit like when drivers like to hold onto the gearlever of cars whilst driving causing premature shift fork and cone wear.

[Dagwood 57]

Surely a gearbox that is so sensitive to oil level and offers no means to check said oil level in a typical (and within spec) installation is not fit for purpose.

 

Seems completely daft - it's 2016 for goodness sake.   

[ballystick 73]

I agree, also seeing how they apply silicone sealer it backs up the case.

[Guest 000]

With reference to my OP, the problem with an inclined gearbox installation where the dipstick is at the front of the box is that more oil is needed to get the oil to the dipstick mark. I sought clarification from PRM and they have now responded.

 

The following from Martin Harris, Engineering and Service Manager, PRM-Newgate Ltd.

 

'In response to (2), you are quite correct in your assessment & the forward clutch is submerged further below the oil level by the approximate amount you have stated. I would recommend that you decrease the oil volume to 0.30/0.35 litres capacity.'

 

This seems a sizable reduction in the oil volume of 0.56litres as stated in the manual, but I suppose they must know what they are talking about. It must also raise the issue of liability whether outside the guarantee or not. How many people follow the manual's instructions and fill the box to the dipstick line regardless of inclination (not mentioned) and damage their box as a result?

[Island Time 1,245]

I'd do exactly as he says - you have his instructions in writing from the manufacturer.... More than ample justification in the even of a warranty issue.

 

I have a saildrive - so no angles to worry about. On other boats, I have always filled to the dipstick line. Now you have given me yet another complication to think about! An why is this NOT in the manual? Seems to me virtually every vessel I've ever seen with a shaft drive has an angle on the gearbox. Stupid.

[wheels 543]

Yep!

Chris, make sure you print that and store it away safe
By the way, is there an oil cooler? A certain amount of oil is going to be taken in filling lines and cooler if there is one. So make sure measurements take that into account.
The reason why you can get away with less oil in a mechanical box is that the oil gets picked up by the submerged gears and then gets taken around the box. This is also the reason why these boxes can be left spinning while sailing with no harm, where as some Hydraulic boxes can't.

Thanks erice, i had never seen that info before, but i see it is for a Car Gearbox and that would be why.

[Guest 000]

No, no oil cooler fitted.

On the one hand, I'm very pleased to have an answer on oil levels from PRM - straight from the horses mouth. On the other hand I feel a faint disquiet that I am directed to reduce the oil level in the box by almost half.

I guess there's just no pleasing some people....

[erice 732]

if it's just a simple

 

'pool of oil at the bottom'

 

that's all you need

 

,  

 

if you're worried the factory's reduced oil amount is doing too much work

 

replace the atf more often

 

smaller amounts are at least easier + cheaper to change

[wheels 543]

Ummmm....yeah....not quite so simple as that erice. The gears produce heat during normal operation and the role of the oil is to get that heat away from the gears to the outside of the crankcase. Unlike an engine that has water, the oil in this case is doing 100% of the cooling. So the box is relying on the oil to contact all the heat producing parts to take that heat to the casing.
    If it were not for two points of concern, I would recommend two things to do. Either add a small amount of Moorey's, which results in increasing the EP strength of the oil. This causes the oil to become tacky and the oil suddenly picks up around gears and flows over everything. Or to use Amsoil ATF. But the two reasons why I would be hesitant in this instance is due to the Box being under warranty and having had issues with the last one and the other is that the boom of words state not to use top quality oils like synthetics.

[Island Time 1,245]

Yep, I agree with Wheels. Do EXACTLY what the factory have told you. Then it is their issue if you have another problem. Don't listen to all the "experts" on the web (including me!). Do what the factory have told you or risk the (no warranty) consequences.