Saturday, March 1, 2008

Flywheel Mounting, Part 1

While waiting for various parts to show up, I've been thinking about the problem of mounting the flywheel to the motor shaft. Azure Dynamics helpfully points out that the splined shaft has a matching hub, available from Hub City, part number 0332-00034. I found it in their online catalog:


After finding a local retailer who carries and special order Hub City parts, I found myself the proud owner of two of them (one as a spare). Here is one, in person:


It will go onto the motor shaft. Here it is, next to the shaft, for scale:


Here is a test fitting on the shaft - it fits *very* well - it takes a fair amount of steady force to remove it after sliding it on gently:


The plan is to find a local machine shop who can turn this into a taperlock hub. Just as in the Electro Auto kit, the idea behind a taperlock is to have a tapered hub that fits into a conical opening in a larger plate, and then the compression of the two with 5 screws also compresses the hub around the shaft. It is going to be tricky with the splined hub, since the splines will tend to resist compression. Here is a first pass drawing of how the hub will be modified:

I need to think about it some more - specifically:
  • Will the two cuts allow the hub to compress sufficiently against the splines?
  • Should the cuts be partially through the front of the hub (as shown), or through the back of the hub (with the uncut part machined smaller yet), or would a single cut yield better results?
Maybe once I find a good local machinist they can help me answer some of these questions.

One other option rather than turning the hub into a taperlock is to tap a center hole in the shaft, and then use a center bolt on a machined plate to keep the hub in place. The drawbacks are that any torque opposite the thread direction will tend to make the bolt want to come out. It is possible that the splines will prevent this (especially if the machined plate is also bolted directly to the hub, in addition to the shaft). But the possibility concerns me, since with regen braking, torque is applied in both CW and CCW directions on the flywheel.

13 comments:

David said...

I believe you are correct in thinking that the splines on the shaft are going to resist the compression effect you're going after. I'd also be concerned with your adapter cracking out over time, as the splines might act as little wedges which want to force your expansion slits open as the torque changes. In the EA version, the adapter sees a more uniform applied torque.

I'll think about it and get back to you.

Dave

David said...

A little more digging resulted in more head scratching, and then a possible revelation...it almost sounds like EA is recommending using a taperlock directly onto the splined shaft without a hub adapter. You'd get more contact force over a smaller contact area, but you'd also avoid the potential nastiness of custom machining an adapter to mate the proverbial square peg into the round hole.

It might be worth a call to EA.

Dave

Ross Cunniff said...

Thanks, Dave. This is not an EA kit, I'm making up as I go. I'll dig a little more; in the meantime, here is a pointer to option 2:
compressing the *rear* of the hub
, and only tapping the compression bolt holes 1/2" in. Also, this puts 4 slits in the compression area, distributing the compression better (but obviously leading to more opportunities for cracks to form).

I'm not sure how much cracking is really going to be an issue - the tolerances between the hub and the shaft are extremely close - there will not be much if any actual compression, just lots of pressure / friction.

Ross Cunniff said...

One last thought - what if I just cut the tapered hub into 4 90-degree wedges, tap 2 holes into each (leading to 8 compression bolts total), and do it that way? I think that would avoid all of the cracking issue, and would yield nice, even compression all the way around.

Kind of like a drill chuck in reverse. I'll post a picture later tonight of this idea.

Potential drawbacks are that it could be difficult to ensure that the wedges are compressed evenly, but that could probably be avoided with careful use of a torque wrench in applying the compression.

Roy Cunniff said...

Dad said,

Your latest version is fraught with danger. Every hole you drill into the adapter/spline creates stress points which could lead to premature failure.

I did some visualization, and several potential solutions depend on what is on the end of the adapter/spline. perhaps you could use two tapered wedges, both inserted into the adapter to bring pressure on the spline. The wedges could be trapizoidal, with the wider portion at the spline end.

Another possible solution would be two spring steel fittings, anchored in two small holes for each in the spline end, and hooked over the end of the spline and then inserted in the adapter in a small grove. This solution implicitly assumes that for future actions you might want to be able to dissasemble the adapter for whatever reason.

Ross Cunniff said...

For what it's worth, the outer hub will be about 4 inches in diameter, tapped to match the flywheel (pictures of the flywheel are here and here). The flywheel could in fact be used to hold the trapezoidal keys in place - that sounds like the most promising avenue so far (although lugging the 230-pound motor to a machine shop to drill & tap the shaft does not sound like fun).

Roger Daisley said...

Drill and tap the hub for the flywheel, then weld the hub to the motor shaft. You may have to weld a flange on the hub to provide a larger area for mounting the flywheel. (Possibly use the Jeep crank flange.)

Roger Daisley
Pullman, WA
http://www.96-volt.com

David said...

It still seems to me that EA has a solution already engineered for the AC55. To quote from the EA website, "The AC55 motor comes ONLY with the splined shaft and is used with our taperlock adaptors." Of course, they refer to this as being a "Solectra AC55" rather than an "Azure Dynamics AC55." However, the pictures look identical, the specs seem identical -- sure looks and sounds like a duck.

Seems like if the solution is already out there, why come up with something new?

rheuckeroth said...

Did you save money by buying direct from Azure vs Electro-Automotive? I am considering converting an SUV once our current lease is up on our SUV.

Ross Cunniff said...

The cost of the components is the same ordering from EA or Azure. But EA does not have a Jeep prefab kit, so I'm going to roll my own.

rheuckeroth said...

I was hoping to eliminate the transmission completely when I convert my SUV. The AC 55 produces over 200 ft-lbs of torque from just about a standstill. With a 3:1 differential thats 600 ft-lbs at the wheels. Plus that should get you to about 50 MPH before you start to get to the torque fall off point of the AC 55 curve.

Azure says on their website that the AC55 is designed to be coupled directly to the drive shaft of a 5000 lb + truck. Its a completely different animal than the AC24 that you have in your Porsche. The AC24 was designed to go into a Geo Metro with a 10:1 gearbox.

Doing this would allow the motor to me mounted where the transmission normally is. It would be easier to directly couple the motor to a Universal joint. This would free up the entire engine bay for batteries, or in my case a series hybrid configuration.

Ross Cunniff said...

I considered a direct drive as well. But I think the acceleration will just be too sluggish. The key factor is actually, I think, shaft horsepower, which peaks around 2000RPM for the AC55. Given that peak, see this spreadsheet. It shows percentage of max power output available at various speeds in various gears (the red boxes in 1st gear are greater than 8000 RPM which is the AC55's mechanical limit).

The 4th gear in the Jeep is 1:1, so that's the kind of acceleration you'd get without a tranny.

rheuckeroth said...

I did some calculations based on a 3:1 drive ratio, and you are absolutely right about the acceleration being slugish. The AC55 would take nearly 30 seconds to accelerate to 60 MPH assuming a 5000 lb after conversion curb weight. I don't know how Azure claims that it can be coupled directly to the drive shaft with a 3-5:1 final drive ratio.

I think a continuously variable (CVT) transmission like on the Nissan Murano might work well with a flat torque curve like the AC55.

Look forward to seeing your project progress!