Would solid motor and tranny mounts stop it from rotating and flexing, or would it make it worse?

 

Probably make it worse. If you have solid motor mounts, the front cover will be near impossible to move, but that doesn't stop the rotor housings from continuing their movement. If anything, it makes it even more possible.

I remember reading something (probably on rotaryaviation, I remember it was about a 12a in a plane) that talked about the tension bolts being coated with heat-shrink. The reason that they did it was because the tension bolts are anchored on top and bottom, and don't actually have any contact with the housings on their way through. They used two coats of heat shrink in the middle to partially anchor the bolts in the middle, to stop them from resonating like piano strings and snapping at high RPMS.

Now you're all thinking "so what does this have to do with our problem?". Well, the rotor housings are moving in part because there is enough play around the bolts to allow movement under pressure. I think if we took up that slack around the bolts, it might help a little bit towards preventing the movement of the rotor housings.

Think of it this way. If you stuck a 3" thick pole into the ground and dropped a bus tire over it, you could move that tire a foot to the left or the right before hitting the pole. If you have that tire mounted on a rim, and the pole through the centre of that rim, you can move the tire maybe two inches to either side before contacting the pole. The pole is a tension bolt, the tire is the hole in the rotor housing. The rim becomes whatever bracing we use (be it heat-shrink or whatever) to stop movement.

Now if this makes ANY sense to anyone, I'd like to hear it... because now that I've written it, I think that it might be hard for people to understand. I tried my best though.

Basically I see the problem being solved by either taking up the space around the bolts that allow the housings to move indipendantly, and/or increasing the pressure holding the housings to each other more so that the friction between them prevents movement. Anything else wouldn't address what allows this movement to happen, but would just be a way of putting less stress on the area (like low-comp rotors would).

And of course, any way of bracing housings to each other on the outside would *help* but only be a partial solution. Then again, anything helps.

Jon

 

I definitely understood the tension bolt void between the housings and the tension bolts themselves - one of the reasons some people dowl the areas in question from flexing as much.

Viper is definitely looking at this problem insightfully. A question then needs to be asked, are the tension bolts when "somehow" thickened (be thicker themselves or shrinkwrap or other method of thickening) able to take side forces without breaking or get workhardened in the process then themselves?

Or is this a do-able solution of simply making them "take the load" of the rotor housing flexing outward by some means of thicker tension bolts to close the gap. I just see that aluminum housings are weaker, and or the side housings were the tension bolts pass through fracturing on the edge were it enters/meets the rotor housing.

 

The shrink wrap on the tension bolts reduces the 'piano' string vibration in the bolt. When the bolt vibrates, all of the flexing stops where the bolt is threaded into the front iron. Because of this, this is where all of the stress is concentrated and where the bolt will fail.

The shrink wrap does not strengthen the bolt per se, but it will reduce fatigue failure.

Thickening the bolts will help reduce the vibration, but unless there is direct contact with the housings and irons, they will not reduce or prevent housing expansion.

I have wondered why Mazda didn't groove both the housings and irons, and use a more suitable cooling seal, to prevent the flexing you are encountering. It may be simply, that due to the differential rates of expansion between aluminum and cast iron, that a seal would not hold, or the groove edges would fracture prematurely.

 

That was the idea I was aiming for. Beef the bolts up enough so that the rotor housings cannot move back and forth when the front, intermediate and rear housings are not moving.

Like a toothpick holding my club sandwich together, only tension rods holding my rotary engine together.

I was figuring that the movement we were talking about was because of the increased pressure from the explosion in the rotor recess near the spark plugs due to the turbo. I wasn't thinking of thermal expansion as the reason for the movement - all our cars have that movement. I figured that (with my poor understanding of physics) when the explosion happens it pushes the rotor and the housing in opposite directions. The rotor is movable, so it is moved by the explosion, turning the e-shaft, making power etc... We tend to think of the combustion chamber as not movable, but in reality, enough force from that explosion will cause that rotor housing to want to jump towards the spark plugs. The only things stopping it from doing so are the tension bolts and the friction between the rotor housings and the intermediate/endplates.

So, in essence, every time there's an explosion the rotor housing tries to "jump" out of the sandwich and is stopped by either making contact with the tension bolts or encountering enough friction from the sandwiching of it.

Jon

 

dowell pinning anyone?

 

I'll take your sandwich analogy and raise it.

Make ham and cheese sandwich without condiments or spread, put a hand on each face and twist. Now make the sandwich with way too much mayo or mustard or whatever condiment you prefer. The sandwich will twist and slide all over the place. This is the extra twisting that occurs in the stack due to the torque with the increased boost you are running.

Not that this analogy will make a lot of sense, but friction, (hold the mayo) and the dowel pins, (your toothpicks) hold the stack in alignment, the tension bolts just squeeze it together.

You are correct that the housing expands outward at the combustion chamber, and this is what causes the failure you mention. The only way to prevent that particular event is to dowel the motor in those areas.

I suggested the re-enforced baffle because this will reduce twisting in the stack. This will help part of your problem. That adresses one of the 3, twisting, dis-similiar metals and expansion of the combustion housing. This by far the cheapest DIY option.

Tension bolts have narrowed shanks, the threads are a larger diameter. To get direct contact between the stack and the tension bolts, they would have to be machined differently. Widened shank. To get the fitment you want, between the bolts and stack, the stack will need to be line bored to match the bolts. That may be more effort and expense than simply adding dowels.

 

i think this is true, solid tranny mounts would (in my opinion) help significantly, not for the expansion that pushes the rotor houseing out but for the twist that is experienced throughout the motor, if you look at where the motor is braced, like previousely staded, the 12a motor mounts are on the front, which means that if you have old sloppy tranny mounts there is allot of stress put on the 12 a block from the driveshaft. i think that this definately has some merrit, if the stock tranny mounts and possibly engine mounts were replaced that would lessen the stress put on the motor core.

how much doint this would lengthen your engine life is argueable, but i think a combination of these things would help,

about the bolts being shimmed possibly by heat shrink... i could definately see that working, but why not see if the 13b or turbo bolts are larger, i realize that the 13b motor is 20 mm's longer but if the bolts are stronger and thicker than the 12a would it be so hard to put a series of washers to counteract the 20mm difference or a plate spacer of some kind.

i was also wondering about the possibility of ecentric shaft flex adding to wear in the side houseings in the motor, because i read, i believe in the "building a high output 12a" thread, which has been stickied and put in the archive, that the 12a experiences some eccentric shaft flex causing premature wear in the motor.

just ideas i thought of after reading the thread.

 

20b tension rods are thicker, FWIW some of the shops over here use them to stop rotor houseing drift, is that what you are reffereing to?

you need to have the threads reamed, and retapped up a size to use them though. IIRC they barely fit though the stock rotor housings on a 13b therefore reducing movement, biggest problem is you must put antisieze liberally on the safts to keep them from corroding into place as they are that close.

kenn

 

I like the responses I get from this thread. I should do the thicker tension bolt option, AND dowling! Those are both very spendy options to consider. And its no surprise why Mazda put the engine mounts further back with each generation to reduce twist as a problem and added extra thick tension bolts for 20b's. 12a is just designed for stock normally asperiated usage. I also realize the engine twists because of the torqueing the engine trying to spin under hard load. Keep the sugguestions coming

 

If you went with solid front mounts, and designed a rear mount that bolted to the engine, like a half circle, at the tranny, and then to the frame on each side, you shouldn't have any twist at all.

The 13b bolts wont work. By the time they were shimmed, you'ld be into the flywheel.

 

Man just get the motor pinned it is the simplest solution. Dowell pinning the motor isn't entirely needed if you get your motor properly tuned. Preignition and detonation is what causes the motor to twist and move. I've seen plenty of motors pushing over 20psi boost without being pinned it's all in the tuning. If you don't detonate your motor won't twist but for most of us who don't have great tuning skills you add dowell pins to your motor and that's why my motor is pinned. Bigger tension bolts aren't needed if you add extra dowell pins believe me on this one you'll be wasting your money and time. I'm lucky I have a friend that pins the motor for me for free but the money spent on it, if you have to pay for the service, is well worth it. If your going to build a motor with some substantial amount of power spend some money it will last longer. Get hardened gears, race bearings and properly clearance everything. Also get the assembly balanced and when it comes to my motors I don't care if the assembly is matching and from the same motor I still pay to get it balanced. The more time you put into your motor, which also means more money, the better and longer it will last you. Just my opinion.