Minute Stroke, Low Friction, Outdoor Shear Stop 2

[swearingen]

The situation:

A rotating piece of equipment (4 rpm) sits on 4 feet. Each foot sits on 2 load cells. The cells are fine for vertical load, but don't do well for horizontal (even with the manufacturer's stops installed). In a hurricane wind zone this poses a problem. Essentially, I'm looking to prevent this thing from moving sideways while allowing it free vertical movement to keep the load cells accurate.

I have entertained the idea of multiple panhard bars, but installation will be hairy at best. My idea is to weld a pipe to the bottom of each foot and a pin to the base. The pipe would fit over the base pin snugly, allowing little to no horizontal movement. My problem is how do I reduce the friction of the vertical movement as much as possible while still preventing horizontal movement? Should I use bushings and/or some sort of grease? What do you recommend?

A few other notes: the owner wants it to be idiot proof (I thought of storm anchors, but that would require someone to go out there and install them when a hurricane was coming - nixed by the client). Since it's outdoors, it must not be susceptible to seizing up over time and not require short interval cleaning. Also, the vertical movement of the load cells, as you might imagine, is on the order of thousandths of an inch.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[MintJulep]

The likelihood of making a pipe and pin assembly of sufficient precision to restrict sideways movement while allowing free up and down movement without binding as side loads change, things expand and contract with temperature, etc. seems remote.

Are the legs mechanically attached to the load cells, or do they just "sit" on them?

I would look at some sort of slide pad between the equipment and the load cells. Then you could do a "loosey-goosey" arrangement to keep the thing attached to the building.

 

[limey1]

Can the whole unit be enclosed to protect it from the elements? If the wind gets high enough, wont it affect the load-cells anyway, if its not somehow protected?

 

[swearingen]

The legs are mechanically attached to the load cells. If there was a slide pad on the load cells, I'd still have the problem with restricting lateral movement of the equipment.

Enclosure is not an option. I've come to understand that all of the other machines like this that were done on load cells are enclosed, so I don't have much to go by.

I had not considered the temperature issue with my pin idea. I think that pretty much rules it out. Star for you, Mint.

Looks like I'm back to the panhards, then. Any suggestions? Should I use bushings there as well? How about adjustment for installation? I was thinking of using turnbuckles, installing them, and then welding the body to the shank to make up for gaps in the threads which would produce unwanted motion when the load reverses.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[MintJulep]

With the legs attached to the load cells you need to restrict lateral motion to near zero.

With slide pads you could allow some lateral motion to occur, and have a much more forgiving restriction system while still never applying lateral force to the cells.

Something like:

|
|--- | ---|
| | |
| SLIDE |
| CELL |
-----------

 

[swearingen]

I don't think a slide pad would relieve enough of the lateral load from the cells. What kind of friction coefficient are we talking about? My numbers say it would need to be below about 10%.

I figured the panhard bars, properly fitted with bushings, would provide a very tight lateral fit while allowing very easy vertical movement. Can you see anything that might shoot them down?





If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[MintJulep]

The problem with anything with a turn-buckle or other type of adjustable threaded fittings is that they just scream "adjust me" to the maintenance monkey.

If the geometry works out ok, and you are confident that people with wrenches, but no understanding of the rods' function won't be twiddling with them, or using them as a convenient ladder to get to the equipment, or as a tie-off point for whatever then I guess they will work.

In my experience, anything that requires precision adjustment will at some point be subjected to some flavor of abuse, screwing everything up.

A mu of 0.1 should be achievable. NiCoTef, or Hi-T-Lube or other slippery coating and a bit of grease.

 

[GregLocock]

If you want to use a mechanism then I'd use 4 panhard rods with bushes, or if you want to be swanky you could use a pair of watts links. Typically the torsional rate of a bush is 6 Nm/deg.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SincoTC]

If the vertical movement of the load cells is only a few thousandths of an inch, have you considered using a flexure strip to form a solid state hinge? Absolutely rigid along the length, torsionally light with the correct thickness material (Stainless Steel Sheet) and absolutely no stiction or adjustment problems. They are generally used in small instrument applications but I see no reasons why they can't be scaled up.

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756

 

[swearingen]

Now SincoTC, why couldn't you have posted that a few days ago? That's brilliant! If I used a 1/4" plate, 4" wide, 10" span, bolted on each end, it would take almost 4 TONS of compression while kicking the load cells off by only 5.9 POUNDS to deflect over the 0.013" throw.

Problem is, my idea has already been drawn up and is moving forward. I decided to go with dual turnbuckles in opposition. This gets me around the load reversal-slack problem. The turnbuckles are tightened against each other - when the wind blows from one direction, only one turnbuckle sees the load in tension, while the other one would feel the load from wind blowing in the opposite direction.

Thanks for your post - I might be able to use that idea in the future...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[SincoTC]

Thanks for the star. Sorry it was too late, I've been a bit hectic and only saw your post yesterday.

I hope that the opposed turnbuckles won't fight each other as the temperature varies and that MintJuep's maintenance monkey resists the temptation to fiddle. Good luck!

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756