About that Mark VII melting problem

[StevenAyres]

So I was running a high-speed operation on the VII that took longer than I expected it would, and when I went to reduce speed to shut off it wouldn't move. I shut down, look inside, and yup, the speed-control cam was melted to the yoke on the control sheave. Arrgh!

I've been thinking that the machine's reputation for doing this might be overblown and mostly due to poor maintenance, not enough oiling, like that, and I was feeling a little discouraged because I like so much about this machine, and the fatal flaw of spontaneous design failure would be a major bummer.

But as I was working on replacing the cam I looked a little closer: the yoke bearing was completely shot. Crunchy-shot. It was so bad that the outer race had dug a shiny trench in the body of the yoke. No wonder it was hot!

So I've replaced the cam and the outer control sheave, including the yoke, and the machine is back up and running. Throwing away rare parts is not my idea of a good way to live, so I'm looking at how to go about repairing the old one. The bearing seems to be swaged onto the yoke, maybe done in a press. Has anyone done this operation and got any tips to share?

[lahola1]

Luckily, I have not had this failure but I do have few thoughts on the subject.

Yes, they are swaged in so if you can clean the detents off with a dremel or something and carefully tap the bearing/race out from the back (be careful; people have cracked the hub trying to get the race out).

Now, if you get the race out I would think a machinist could install a sleeve in the damaged hub the correct diameter for the new bearing (if they can install sleeves in cylinders of car engines I would think they could install a sleeve in a SS sheave hub).

On another mkVII I posted rough measurements of mkV and mk VII control sheave hub lengths.
mkVII approx 1 1/16"; mk V approx 1 1/8"; so I would think you could machine a mkV sheave hub to the correct mkVII length but I don't have accurate dimensions including depth in the hub for the bearing seat.

If you get the race out successfully, could you get those dimensions and post them? that would be good info future reference.

[StevenAyres]

people have cracked the hub trying to get the race out

I expect you're thinking of the Mark V setup. On the VII it's swaged to the yoke rather than the hub. The assembly popped out of the hub easily with a drift, a nice plus relative to the V.

Mark VII control yoke illustrating bearing attachment

PXL_20201219_150801736.jpg (42.35 KiB) Viewed 1327 times

Mark VII control yoke illustrating bearing attachment

PXL_20201219_150819337.jpg (40.13 KiB) Viewed 1327 times

So I'm considering how to remove the old bearing while leaving a useful post on the yoke. The first approach I think of would be to cut the old bearing off, reshape the post and the swage the new bearing on again. Second would be to cut it off and reshape, but then tap the center of the post for a screw, making the bearing more easily replaced. I'm not seeing any forces really militating against this, but I'm hoping direct experience can offer some confidence.

could you get those dimensions and post them?

The hub dimension (inner flat to outer edge) is 2.11"; the bearing cut is .22" deep and .742" in diameter.

Mark VII control-sheave hub

PXL_20201219_150726635.jpg (35.41 KiB) Viewed 1327 times

[lahola1]

No, I was thinking of the mkVII sheave. I thought the bearing and cam follower was swaged, dimpled to hold it onto the sheave. I didn't know the post was swaged to hold the bearing onto the follower. I thought it was probably pressed on.

I think either repair you mentioned would work. I know the cam follower is easily drilled and tapped so I think the post is also.I tapped my cam follower to attach a thermocouple to monitor my sheave bearing temp to try and avoid the problem you had.
Here is a link to that thread . viewtopic.php?f=10&t=20541

great that you got the bearing out without damaging the sheave and got measurements. now if someone has a mkV control sheave without a bearing that can be measured and post their measurements we will know if an almost irriplaceble mkVII sheave can be replaced with a mkV sheave.

Good luck on your repair.

[JPG]

I like the tap/screw approach.

FWIW the bearing is also swaged to the hub.

[lahola1]

Some interesting stuff here. I finally see the color that the cam follower turned to that caused the plastic cam to fail. It looks brown, almost purple which would be 480 deg to 540 degrees which is where I would expect the nylon cam to fail.
A couple questions:
When I measured my hub length, I measured approx 1 1/16"; only the smallest dia length(appox .900" dia) . Your 2.11" length sounds like the total length of the sheave. Is that correct?

When you get the bearing off, what is the dia of the follower post?

[cham-ed]

there are two ways that yoke can go on and one is wrong.

[JPG]

The bearing goes in only one way. Yoke to cam is when you have one correct and one wrong way.

[StevenAyres]

Your 2.11" length sounds like the total length of the sheave. Is that correct?
When you get the bearing off, what is the dia of the follower post?

I measured from the inner flat to the outer rim of the sheave hub, so total length, right. I get 1-1/16 where you measured as well.

I don't have it apart yet, but the post on the yoke looks like 1/4", making the bearing a standard 1/4,3/4,9/32. Does that fit with experience here?

[lahola1]

Thanks for the dimension update.I'll keep them for ref.