Press-fit or Threaded Connetion 2

[Brent Boisvenue]

Hi everyone,
I'm designing a type of ram for a high temperature furnace application requiring linear guidance. I have two linear guides on the top of the machine and a "push rod" at the bottom connected to a pneumatic cylinder (see attached image). I have guide tubes (grey) projecting out the back of the mechanism, housing high temperature Thermalloy bushings. The guide rods (green) project through the flange and into the furnace and are supported on the back side using the guide tubes that are mounted to a 1" flange to the burner.

I have three methods I'm considering for attaching the guide tubes to the flange:

[li] Threaded connection with shoulder (current design shown in screenshot)[/li]
[li] Press-fit[/li]
[li] Slip-fit hole with bevel weld ground flush on back side[/li]

Given that this is a linear guidance application, I'm looking for something that can provide decent perpendicularity to ensure alignment, although I've build in a decent amount of slop since the application doesn't require any precision. My first instinct was to use a threaded and shouldered connection (shown in the attached screenshot). However, one of my colleagues is a bit concerned that I won't get the requisite perpendicularity from the threaded connection.

Any recommendations?

 

[SwinnyGG]

You'll get good perpendicularity as long as the shoulder geometry is correct.

If you truly don't need a high degree of precision, that's the way I would do it, if for no other reason than the fact that it's likely the least expensive and the easiest to assemble (and disassemble later if that's a need).

Welding would bring some precision problems of its own, and a press fit is great IF the parts are easy to press together... looking at this thing I'm not sure that's the case. If you went with a press fit, does this thing fit in a press?

You may want to keep in mind that your guide tubes are going to take some load normal to the long axis - they may need bracing outside of the threaded (or whatever method you settle on) connection to the flange.

 

[Brent Boisvenue]

@SwinnyGG,

Thanks for the advice! You've pretty well summed up my concerns on the other connection methods.

I've already accounted for the loading on the tubes. I'm good for strength on max cylinder exertion at the worst point of contact on the ram sleeve.

 

[wde0665]

Agree with user SwinnyGG from a maintenance perspective. Figure this thing is going to be worked on in the future, so which is the least time- and material-cost method to restore to like-new performance?

Perhaps bringing to light a long-standing gripe - the stuff which performs beautifully from an engineered view, but a total bite in the shorts from a maintenance perspective to keep it operating that way.