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Motorcycle Discussions => British Bikes => Topic started by: JFerg on October 03, 2021, 06:35:42 AM

Title: Not an oil thread
Post by: JFerg on October 03, 2021, 06:35:42 AM
I am wrestling with a perplexing problem.  In a total loss machine, I can't get the oil to flow, by gravity, through a perfectly clear, newly made, copper line.

Connect to the tank, turn on oil, wait, and nothing comes out at the pump.  Can't even draw some through by sucking on the line.  The initial line had four horizontal turns in it.  Take it off, blow through it with ease, clearing only a small slug of oil.  Turn oil tap on, oil flows freely.  Blow through pipe very easily.  Either way.  Put line back on, no oil flows.

So I remake the line replacing the four horizontal turns with two sweeping turns, ie; pulled out so there's always a clear downward path.  This worked, initially.  Today, however, I could not get any oil to flow.   I cut the sweeping turns out and replaced them with a length of clear plastic hose, and guess what?  Oil flows sweetly.  I forgot to turn the oil off and the pump filled, which has never happened before.

Not sure what the oil grade is; I use "smokeless two stroke oil", the cheapest I can get.  It's a light grade and will leak out of anywhere it can escape through, but it won't go down a copper pipe that I want to to go through.  Also, the "smokeless" bit is a lie.  The oil tank cap is vented.  It's 1/4" OD copper tube, which is 4.7mm ID.

What is going on here?  Plainly there's some kind of drag, but I'd have thought that would only affect the contacting molecules.  The flow rate I need is only modest, but I can't get a cracker.  Any ideas as to why this is so?
Title: Re: Not an oil thread
Post by: murdo on October 03, 2021, 07:27:54 AM
Just to be a 'devils advocate', what happens with a heavier (say 30wt) oil? Does this flow any better?
Title: Re: Not an oil thread
Post by: R on October 03, 2021, 07:45:34 AM
Run some petrol down it, and see what happens ?

You may even flush out those beetles that have taken refuge in there ...
Title: Re: Not an oil thread
Post by: cardan on October 03, 2021, 08:34:54 AM
Remake it in 5/16" - rare to see 1/4" for oil.

Title: Re: Not an oil thread
Post by: JFerg on October 03, 2021, 11:12:10 PM
It's in 1/4" because I have plenty of it, but what is the physics behind this?
Title: Re: Not an oil thread
Post by: cardan on October 04, 2021, 03:13:05 AM
Hi JFerg,

It's viscosity. The oil sticks to the wall of the tube, where it is stationary, then moves faster as you move away from the walls. In a skinny tube, the bit in the middle is still moving only very slowly, but as the tube gets bigger the oil in the middle moves faster.

Unlike petrol pipes, oil tubes very rarely had horizontal coils in them as this slows the oil even further. In veteran days good quality bikes like Triumph and Rudge used a short rubber section in the near-vertical oil pipe to stop it fracturing due to vibration, but to let the oil run down at reasonable speed.

With a manual hand pump on the tank a skinny tube is not too bad because you can force the oil down the pipe into the engine, but with gravity feed to the pump (really a "metering device" on vintage engines) you need a big near-vertical feed line from the tank. If you have a tap in the line, make sure it also has a big hole in it, as the flow is determined by the skinniest bit.


Title: Re: Not an oil thread
Post by: JFerg on October 04, 2021, 09:51:21 PM
All fine, Leon, but the question remains.  Oil will flow down an essentially vertical pipe freely, can be sometimes persuaded through a pipe with two drawn out turns (and 10" greater length), point blank refuses all coercion to pass through a pipe with four turns in it, yet belches from half a thou on the pump end plate.  Admittedly that is under pressure, but you see my point.

The pipe has a 180 thou ID, oil has no surface tension, is low viscosity in this case.  A restricted flow I could understand, but none at all needs an explanation.
Title: Re: Not an oil thread
Post by: R on October 04, 2021, 11:30:00 PM
oil has no surface tension, is low viscosity in this case. 

'Low viscosity' is relative. Try flushing some petrol down this tube,  and see how it goes.
I'd agree, I've not seen coils in an oil supply line.

'Oil has no surface tension' needs some comment too.
An oil drop that has fallen onto some types of surfaces has quite a high rounded domed shape, so clearly has much surface tension.
In a narrow bore tube, unless its under pressure, it will not like to flow much.

I've got an oil pressure gauge on the end of a long thin extremely small bore pipe.
But it is under pressure ...
Title: Re: Not an oil thread
Post by: cardan on October 05, 2021, 03:11:53 AM
"... refuses all coercion..." is the key. There is (almost) no coercion!

Without a pump at the top pushing the oil down the tube, there are only two forces acting on the oil: the air pressure on top of the oil in the tank, and gravity.

The air pressure on top of the oil is 14.7 psi (atmospheric pressure). The inside diameter of your tube is 4.7 mm, so the area of the tube is only 17.3 mm^2, or 0.027 square inches. So the "force" due to air pressure is .027 x 14.7 = 0.4 lb. Not too bad - you can think of this as having a little plunger in the top of the pipe, weighted down with 0.4 lb. BUT if the bottom of the tube is open to the atmosphere, there is also 0.4 lb pushing UP the pipe!!!

Sucking mightily on the bottom of the tube helps get the 0.4 lb back - this is what the oil "pump" down the bottom does. But no matter how well it works, it will never give more than 0.4 lb to push the oil down the tube from the oil tank. An oil pump can PUSH but it can't SUCK the oil.

Gravity helps too - you get the mass of the oil in the vertical part of the pipe, plus the mass of the oil in an imaginary column of oil 4.7 mm diameter in the oil tank. This is not much! Every 5 cm of oil in the pipe is a gram or so!

In summary, there is not much pushing the oil down the tube.

The viscosity of the oil acts against flow - think of it as friction. As I noted above, the oil at the pipe wall is stationary, then "layers" slip past at progressively higher speeds as you move towards the centre of the pipe - so called laminar flow. Thin oil (say kerosene/paraffin) is not very viscous so it will flow well in a skinny pipe, but as you get thicker the flow slows to nothing - or at least almost nothing. Effectively this is due to "friction" between the layers of oil flowing in the pipe.

(I bet if you leave the tap open the shed floor will be covered by morning!)

Anyway, horizontal parts of the pipe contribute nothing to the gravity + air pressure pushing the oil down the pipe, but they do add to the friction trying to stop the flow.

The answer is a larger pipe (5/16" vs 1/4" is about 50% more surface area) to reduce the effects of "stiction" on the walls of the pipe and to increase the mass of oil pushing down the pipe, and to minimise horizontal sections of pipe, which slow the oil.
Title: Re: Not an oil thread
Post by: john.k on October 18, 2021, 11:18:48 PM
This remind me of an incident with bitumen.....which has a very high viscosity,especially in cold weather......for instance you can form it into a ball like putty...if you dont mind black hands.........Anyhoo,picture a old  tanker with say 1 ,000 gallons of bitumen ,sitting in a field .....and someone accidentally knocks off (breaks) the valve ....nothing comes out ,and in the following weeks a tiny wire of black goo reaches the ground unseen......five years later ,there is a pool of black goo under the long grass over 100 yards wide.....oddly enough ,it doesnt seem to hurt the grass.