NA Engineers, help!

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
I have this table that is designed to tip. It pivots at a hinge point and one end goes up, one end goes down. Then, a person is suppose to clamp it in place. The problem is that the clamping mechanism is not clamping adequately and the table will slide down despite normal clamping torque being applied by your average person.
I think we need to beef up the 'μfrict-sliding' or whatever it is, between the stationary portion (the rod) and the clamping portion (the bullet). That's where you are needed.
What can I do to the face of the bullet to increase the friction between it and the rod? (see photos) Should I make the bullet a sharp point? Maybe take the dremel and make a cross hatch of the flat surface of the bullet? Keep in mind that any alteration may increase the friction so much that the table will no longer slide up and down freely; an undesirable adverse effect).
I really don't want to mess with the rod. It is less accessible and I'd resent messing with it's dimensions.
Outside The Box thinking might include putting something between the bullet and the rod.

Accepted ideas will have results posted within 24 hours (as I gotta get it fixed) - what better immediate internet gratification (or shame) can be had?!

Thanks is offered to all.
Pics to come. hopefully....
 

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I'm no train driver but...

What about just making the handle longer to give you more leverage? A "cheater" pipe?
 
Or adhere some sort of friction material to the tip of the bullet.
 
This thing looks pretty beefy. How much weight is it supposed to hold? Is that what stripped the friction mechanism in the first place? Overloading it?
 
The most we've had on the table is 150lbs. The weight is centered, otherwise the table would tip over even when locked down. So, it does not have even half that weight at the very end of the table. It is supposed to be designed for that weight. There is a groove in the rod which the bullet had worn into it if that is what is meant by stripped (the threads are fine).
 
If there is a parts manual, check to see if there is a misplaced part either between the threaded portion and the cylinder (maybe a steel ball) or a friction element that would normally press on the sliding rod.
Although it looks like there is plenty of thread there may not be quite enough push if something is missing. Also make sure the contact points are free of sliption-inducing greases.
 
I don't understand how the "bullet" has grooved the rod, Dave. The face of the bullet seems to be flat but you don't have a good shot of it. Is it pointed?
 
Do you need infinite variability in slope?
I know you said you don't want to mess with the rod, but it might be worth it to take it to a machine shop and have them drill a series of pockets along he length, and have it fixed once and for all.

In the meantime, how about a bullet of a softer material, like brass or aluminum for better grip? Maybe a soft metal disk between the bullet and rod? Find a brass screw or bolt of appropriate diameter, and cut a slug 3/8" long or so and put it between the bullet and rod.

A sharp point won't work, it'll only decrease the surface area. You could try drilling a cup in the friction end, similar to the kind found on set screws, and sharpening the edge of the cup a bit.
 
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Looks like a British design....**** poor friction design.

It needs a mechanical lock of some kind....not friction. I'd grind a grove at locations where I prefer the settings. The bullet will lock into the grove vs only friction.
 
How does the bullet connect with the handle? With the groove worn into the rod the distance you have to travel to get a good friction lock has changed by that much. Is there a way to shim the bullet out by that distance?
 
BTW Dave, I think putting a sharp point on the bullet, like you asked about in your OP is the opposite of what you want to do since your dealing with hard, not soft materials. You want more bearing/friction area, not less.
 
EppyGA said:
How does the bullet connect with the handle? With the groove worn into the rod the distance you have to travel to get a good friction lock has changed by that much. Is there a way to shim the bullet out by that distance?
Click to expand...
If the handle isn't "bottoming out" that shouldn't matter. The groove itself may be the issue though...reducing the friction. We don't have photos from and angle that tells us this though (that show the face of the bullet).
 
In the last picture, the one with the handle and short rod. I'm assuming the short rod is what contacts the other rod's flat surface and is supposed to prevent movement. I would attach a material to the friction end of that little rod. I would use brake shoe material. For proof of concept I would epoxy it to the rod, if it works I would try a more permanent solution.

Material I would try: http://www.mcmaster.com/#60895k13/=11pfatn

How I would get a round piece: http://www.mcmaster.com/#hole-forming-punches/=11pfbmj


If it works, you could try it for a while and see how long it lasts, otherwise I would have another small rod made with a counter bore to better secure the friction material.

In use, make sure you loosen the handle completely before moving the table. Also the surface of the large bar would need to be smoothed if dinged up. Good luck.
 
Another idea...

If the groove is what's causing the issue and if the handle isn't bottoming out (if it is, then that's likely an easy fix, but if it's not) then...

You might have someone fab a secondary friction lock that slides over the rod. This could be made out of a short piece of schedule 80 pipe with a hole drilled in it, a nut welded to it, and a hardened bolt (grade 8) with a "t-handle" welded to it.

Take the nut off of the bottom of the rod, slide this contraption on (after you paint it a sterile looking color) and use it as a secondary lock. Just slide it up against the "cross pipe' when the table is at the proper angle and lock it in place.

Should be pretty inexpensive to make.

You'd just have two handles to twist when adjusting the table angle instead of one.
 
The brake shoe material is a good idea, but I think it would crush and crumble from pressure after too much time. I'd try brass first.
 
Contact the company that made the table and ask what part(s) need to be replaced to prevent the slipping.
 
This is the kind of brainstorming that helps, thanks guys. I can't address all questions, here are the important ones I think:
-the table is orphaned, probably >40 years old so no parts, no manuals - we iz on our own.
-the bullet appears to have had a point on it as their is a witness mark where someone ground it flat and the groove worn into the vertical rod is about that big, goes about the distance that the table normally travels. (the groove is also not a neat machine groove.
Thanks. Might try a softer metal first.
 
I'm used to these tilting tables being supported by a leg that swings out underneath the "folding down" piece of top. When the keg is out, the too can't move.

Sounds like you're stuck with a p!ss poor table design.

An option to increase friction would be to add a piece of copper or brass to the end of the bullet, larger in diameter, with a recess on the back side for the existing bullet tip. If there's room to add it . . .

Good luck!
 
Twist the rod 180* so it is thicker on the bullet interface side. Too worn + not enough thread length = your sex table thing slips excessively.
 
The "bullet" thing could be ground to a smaller point (not sharp) and the pressure of clamping would tend to deform the rod just a bit and not let it slip. Take a look at the typical set screw - usually those have a cup point (hollow in the center) or are ground to a truncated cone. You could drill the end of the "bullet" thing to make it into a bit of a cup point as well. You want this part to be hard so you get a little deformation of the part you are clamping to.
Brass would be a good idea if you wanted to make a bearing that slides easily - that's what it's often used for - but I don't think this is your objective.
Cross hatches on the rod could help.
Make sure your threads aren't bottoming out at the same time as the clamp is being made - leave the bullet out and check to see that you can turn it in further than you typically do.
 
I didn't see this addressed in the original post. Do you need an adjustable table? Or can you make do with one or more fixed tabletop angles? If the latter will do, just drill holes or dimples in the metal bar that the "bullet" lands on.

-Skip
 
Probably a little too redneck for a vet office, but I'd just keep two vice grips in there, and clamp them on both sides of the rod once the table is set :)

Certainly would likely damage the rod over time but we'll probably all be dead before it would matter.
 
Probably the thing is worn to the point where the clamping range isn't quite enough to produce the needed pressure on the bullet. The first think I'd do is to put something between the clamp screw end and the bullet. Anything. A #10 or 1/4" nut might work. A ball bearing. It's just a temporary shim to see of this enables you to clamp tightly.

If that's the situation, find a small machine shop who will make you a new rod and a new bullet. They can help you select materials and can make you one or two spare bullets if that is something that wears sacrificially. Even if the shim trick doesn't work, I would still go this route. It looks to me that someone might get hurt if this thing isn't properly clamped. Hence, this is not a situation for a rubber cement and bailing wire home-made solution.

In the short term a couple of split shaft clamps could be used to immobilize the table. mcmaster.com, already mentioned, is your friend for this. The clamps are cheap.
 
On the farm, I'd just cut a chunk of rubber out of an old tractor tire with a hole saw and stick it in between the bullet and the rod. Maybe a slug of metal on the bullet side to make the rubber last longer. If there's enough play in the system to fit these in there;-P
 
Capt. Geoffrey Thorpe said:
The
Brass would be a good idea if you wanted to make a bearing that slides easily - that's what it's often used for - but I don't think this is your objective.
.
Click to expand...

I think you might be thinking of sintered bronze. Brass is soft and sticky (relative to steel) and often used for what the original poster is looking for on high torque pulley to shaft locking applications. Look at most any AG equipment parts manual, there will be a brass plug between the shaft and a set screw.

Bushings sure...bearings no.
 
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Yes, brass would be fine in that application, there are different grades, mainly for machinability, I would go with a harder grade (less lead). You also might want to consider pulling the large shaft out and cleaning up both the shaft and the bore. Someone may have lubricated it at one point causing the problem. There may also be a build up on the bore that affecting its ability to hold the shaft. Don't forget, the contact between the "bullet" and the shaft is only part of the holding power. Most of the holding power probably comes from the shaft being driven into the bore. Runs some emery paper or cloth through it, then clean with a rag and some solvent. Do the same to the shaft, especially the flat. I can't tell if the dark stripe on the flat is build up or plating wearing away. I'm thinking it is build up, if so, I would remove it with emery cloth.

Remember, if it's stuck, force it, if it breaks, it needed to be replaced anyway.
 
Chip Sylverne said:
I think you might be thinking of sintered bronze. Brass is soft and sticky (relative to steel) and often used for what the original poster is looking for on high torque pulley to shaft locking applications. Look at most any AG equipment parts manual, there will be a brass plug between the shaft and a set screw.

Bushings sure...bearings no.
Click to expand...
Brass plugs under set screws are to prevent deformation of the shaft being locked. Not the issue the O.P. was asking about.

Bronze is better for bearings, but in general, many combinations of a hard shaft and soft bushing material will work for bearings.
 
This looks like a veterinary OR table. Do you have to adjust it while maintaining your sterile field? If not, how about using a C-clamp and a short block of wood under the table (as a temporary fix). It would take 2 people, 1 to hold the table in position, 1 to clamp it up. I suppose if you practiced ahead of time, you might even be able to change the position intra-operatively. Obviously someone not scrubbed in would have to dive under the drapes.

You'd be surprised at some of the Rube-Goldberg stuff I see in the "human" OR.
 
couple of notes to keep things on track:
there is absolutely no grease/oil/dirt on any of the parts. you can wipe any part of it with your white sleeved shirt and it would come back clean.
The handle is not bottoming out; there are lots of threads left, I have checked.
Thanks, will check back.
 
Let'sgoflying! said:
What can I do to the face of the bullet to increase the friction between it and the rod? (see photos) Should I make the bullet a sharp point? Maybe take the dremel and make a cross hatch of the flat surface...
Thanks is offered to all.
Pics to come. hopefully....
Click to expand...
File or grind the end of the bullet for maximum surface area contact with the rod.
 
I put one of those expanding lead concrete-wall plugs in and....
 

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.....it was like I greased it. Zero holding power. :(
The original bullet was smooth and flat so I knew that wasnt going to work.
 
So I dremelled rows of 1/16" grooves into the bullet face and.....
 

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....it bites like no tomorrow!!!
Put a person on one end and no slippage!
Now to see if it stands the test of time.
Thank for all input!
 
Great job. Lead can act as a lubricant hence no holding.
 
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