Rolls-Royce Merlin Reduction-Gear Question?

Do I remember rightly that the skew-gear issue was found to be due to order-of-assembly?

As for gear tooth wear, this also brings up the old chestnut of cycloidal vs. involute teeth forms. In theory, the cycloidal form is subject to less sliding friction as the gear rotates, resulting in less wear. People will try to claim that this is the reason my 200-year-old Grandfather clock is still happily keeping time on the far side of the room. In reality, especially with modern manufacturing methods, it makes little difference. Wikipedia has a good illustration of pressure angles and contact points on involute gears

http://en.wikipedia.org/wiki/Involute_gear

It mustn’t half have flexed that bearing panel to cause the skew gears to practically unmesh. It illustrates the forces at work within the engine, sometimes in directions quite unexpected by the designers.

Another description I’d heard of the skew gear failure on the Merlin was “things went bloody quiet up front”.

Anon.

What was the skew gear problem i remember on Merlins ?

The skew gear is on a cross shaft in the wheelcase which, critically, drives both magnetos.
The problem occurred due to the splined quill shaft mentioned above in this thread, which drives the reduction gear pinion.

Bear with me…if the front of the pinion was higher or lower (I forget which) than the back, due to manufacturing tolerances ( the roller bearings at the front and back of the pinion were in different casings), then the crankshaft could be forced backwards with enough force to bend the centre main bearing panel in the crankcase, carrying the thrust bearing, so that the ensuing increased clearance in the bevel gears driving the lower vertical shaft in the wheelcase caused backlash and chatter sufficient to strip the skew gear, which was bronze, causing, to quote Alex Henshaw, “The Deafening Silence”.

It must have taken Royce’s much head-scratching to locate the issue, and many aircraft around that time were lost as a result.

Pete

What was the skew gear problem i remember on Merlins ?

Because they are rotating the gear teeth only bear on a small area and wear can be a real factor. Note this stripped gear (not aeronautical). These wheels are about 15ins dia with a thickness of about about 3 ins. The teeth get worn down and can eventually strip because of excessive loading/ lack of lubrication.

A good dentist is essential.

You are right CD, it is because all the teeth of the spline transmit the torque simultaneously, whereas the gear tooth has to absorb it all on its own.
The cross sectional area of the splined shaft (quill shaft) is about half that of the prop shaft, because it transmits half the torque.
Pete

Prop

But the reduction-ratio is only a factor of the ratio of the teeth on the two gears; I am sure I could get exactly the same reduction-ratio with much smaller gears. The torque multiplication also is only a factor of the ratio of the teeth on the two gears.

Splines and gears are different but in transmitting the power, in different ways, they do exactly the same job. I suppose the main difference between them is the area of contact; in a spline (theoretically) the whole contact surface acts at the same time, in a gear only a tiny area of one tooth has to transmit the entire power of the engine.

There is a big prop cantilevered off the reduction gear shaft, so it is more than just transmitting torque force, deeper in the guts of the engine the splines off the crankshaft are more supported. The reduction gear case, bearings & gear not only support the prop but drag the rest of the engine and aeroplane after the prop.

The centrifugal reaction forces of the prop are enough to spin the aeroplane around its axis, so this force must be spread around the shafts and transmission bearings of the reduction case, the whole design must be sturdy.

But the reduction-ratio is only a factor of the ratio of the teeth on the two gears; I am sure I could get exactly the same reduction-ratio with much smaller gears. The torque multiplication also is only a factor of the ratio of the teeth on the two gears.

Splines and gears are different but in transmitting the power, in different ways, they do exactly the same job. I suppose the main difference between them is the area of contact; in a spline (theoretically) the whole contact surface acts at the same time, in a gear only a tiny area of one tooth has to transmit the entire power of the engine.

Hmm…
I can see where you are coming from but the 400mm is required to get the required reduction ratio.

Also, splines are one thing and gears are another.

Most significant though is that the reduction gear is a torque multiplier (power= rpm x torque in appropriate units).