akldb

I was thinking that the rotor tip seals have large centrifugal forces on them and yet they are needed less at high revs. It should be fairly easy to design small weights near the tips that actually reduce the outward force on the seals as the revs. increase.
Does this already happen? Has anyone else thought of this? Is it worthwhile?
Tony

Pendleton

iTrader: (1)

I believe if you somehow added weight, you would actually increase the outward force. As the rotors spin faster, the G forces are greater. So say a 10 gram weight is spun at 10 X g, the outward force would be 100g. If you spin it faster, it becomes maybe 20 X g, so the outward force is now 200g. (That's using some simplified physics, I'm sure it's more complicated than that.)

JinDesu

iTrader: (2)

You've lost me slightly - you want to add weights to the seals to decrease the "outward" force on the seals?

For one thing, there is no centrifugal "force" in physics. That's momentum.

Huey52

iTrader: (2)

^ There is centripetal force however.

OP: You've completely lost me. Why is rotor apex sealing less important at high rev's?

JinDesu

iTrader: (2)

Yes, centripetal force is the important one. Centrifugal motion is incurred by momentum opposing the centripetal force. I am guessing he thinks that as the rotors spins faster, the centrifugal motion will push the seals tighter against the housing.

akldb

Firstly, I know there is no such thing as a centrifugal force... but that is really only true in an inertial reference frame. I assure you that if you get into a rotating reference frame (which is of course not inertial) you'll "feel" the centrifugal force. For many problems it is easier to move into the rotating frame and just add in the centrifugal and Coriolis forces.

Next the added weights.
Obviously these would not impinge directly onto the seals as this would increase the pressure onto the chamber wall. One could have small leavers to change the direction of the forces to be inward. Then the centrifugal force on the seal would counter the force delivered by the lever from the centrifugal force on the weight.

There is another solution; make the centre of gravity of the seal on the other side of the rotation axis. However, after looking at the construction of the rotor, it became clear that this would be hard to do. It would need a ring all the way around to the opposite side of the rotor where there would have to be a weight to move the centre of gravity across the rotor. There doesn't really seem to be room for this.

That's why I thought of the little lever approach.
One advantage of doing this is that the seal could be improved at low revs. i.e. when starting.
[I just had my fuel pump fuse out yesterday for 30 minutes to get the thing started.]

Also by careful choice of lever geometry one could make the outward radial force of the seal onto the chamber non linear with rotational speed. Lots of options and it could actually be quite simple.

I presume from this discussion that Mazda have not already done this.

akldb

Apex seal at high revs.
The seal is to stop the gas escaping. This takes time. At high revs. the seal does not have to be so good.
Also at high revs. it contributes significantly to the friction. With no control over the centrifugal force the seal pressure increases with revs.

In a piston engine the piston ring force is not affected by the rotation rate of the engine.

Tony

zoom44

i believe he is saying that you could decrease the wear on the seals or housings if you decreased the pressure on them at higher rpms by weighting the seals so the force didnt push them overly hard against the housing.



which is just plain wrong. a complete misunderstanding of whats going on.

9krpmrx8

iTrader: (46)

I like boobies. They are fun.

zoom44

the seals are constructed so that you get proper sealing at speed thanks to the centripetal force. at lower speeds the spring holds the seal against the housing for proper sealing.

this spring is your lever. you have it backwards. you dont need to keep it from sealing "too hard" at higher rpm, you need it for proper sealing at low rpm for starting and power.

Huey52

iTrader: (2)

I still don't get this (perhaps reverse worded from intent?): "rotor tip seals have large centrifugal forces on them and yet they are needed less at high revs"

zoom44

he thinks the seals get too much pressure at high rpm, creating too much heat , causing a reduction in their life expectancy .

Huey52

iTrader: (2)

Oh, OK, so he meant the centripetal forces are large at high rev's, and therefore needed less to facilitate good apex seals. He's actually advocating reducing the seal pressure at high rev's to promote less wear and greater reliability.

Just install ceramic seals and don't worry about it.

I'm certainly looking forward to more precisely worded technical posts once he gets into double digits 'round here.

zoom44

gah.....!!!! advocating! thats the word i was looking for! would have made my whole post clearer. drew a total blank

ken-x8

Correct! Centrifugal and Coriolis forces are indeed real in a rotating frame. The whole "no such thing as centrifugal force" mantra comes from overzealous explanations of why when you twirl a rat on a string, and the rat's tail pulls off, the rest of the carcass follows a straight line in an inertial frame. In the rotating frame it follows a nice curve.

Ken

Brettus

iTrader: (2)

Would be fun to see how quickly whole lot of levers and pivots would carbon up if nothing else ....

paulmasoner

iTrader: (1)

lol, whats essentially simple miniature leaf springs is difficult enough already

doo eeeet

nycgps

iTrader: (1)

x2

what this thread is about again? oh yeah, I remember, it was about bullshit.

DarkBrew

I thought it was a debate about centripetal acceleration vs. centrifugal force cleverly disguised as an apex seal discussion

And a bit about liking boobies

rotarygod

At high rpms, your apex seal springs aren't really doing much. A ceramic apex seal is so slippery yet so light that they run very high spring pressures. This helps with better low to mid rpm sealing. A heavier seal won't help low speeds but will seal better up top, assuming of course you don't get chatter. However things will wear much faster.

MazdaManiac

iTrader: (3)

At high RPMs, your apex seals aren't doing much, either.
You can actually run a rotary motor with no apex seals at all above mid RPM.
Kinda hard to start, though.

PhillipM

Which is what he's trying to say.
Still use the springs for sealing at low>mid rpms, then use a simple counterweighted lever to counteract the force on the apex seals generated from the mid>upper rpm range to reduce the amount of wear on the tip and chamber walls.
Which means you can run springs as stiff as you like for low rpm sealing and counteract the unrequired force further up the rev range with the counterweight.

Whether the wear reduction would be worth the extra few grams at each rotor tip is another debate altogether though...

olddragger

iTrader: (3)

i like thinking people and brainstorming a problem.
i have often wondered about the varying ability of apex/corner and side seals to seal the chamber. Lot more to it than meets the eye, but it is surely an area that improvement is needed?
I am just to dumb to come up withanything.
When I think of a lever--i think of a baseball bat. sigh.
OD

MazdaManiac

iTrader: (3)

Wouldn't it just be easier to machine the seal and the groove as a wedge?
That way, you just limit the outward travel of the seal.

PhillipM

That wouldn't reduce the sealing pressure though as you still need the seal to ride on the wall - so it'd never really hit the wedge....