Brake Pad options

I'm working on a fail safe brake along the lines of an elevator safety brake -- it will prevent a heavy load from falling if the supporting
roller chain breaks. I need brake pads that can be attached to the device that will bear on a large rectangular tube column when activated, 4 pads near the corners of the column. I found some rough dimensions online of performance auto disc brake pads that are about 2" square, which seems appropriate. The four pads will be required to support about 3000 pounds total.
Potential problems with automotive pads? Any suggestions other than auto pads? McMaster has lining material, but I'd rather not design and fabricate something that can be more easily purchased.
--
Ned Simmons

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Ned Simmons wrote:

Most every vertical column safety I've seen has been a ratchet track type of configuration that will positively lock the load from dropping more than the inch or so ratchet spacing in the event of the lift system failing. That would seem to be the best way to go unless your application is significantly outside the norms.
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wrote:

I would prefer that, and it would work if there was room for the track, but unfortunately I haven't found a practical way to mount one large enough to support the load. If I can generate some understandable images of the device I'll post them.
--
Ned Simmons

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Ned Simmons wrote:

Most I've seen were something like a strip of 1/2" plate cut with the ratchet pattern on one side and welded in place on the column. That would only add 1/2" to the column thickness. The safety latches on auto lifts work like this and are rated to hold a 10,000# vehicle over people with just one of these ratchets in each column. You could of course use thicker plate, cut the ratchet pattern on both sides, and have the ratchet pawl engage more than one tooth as appropriate to handle the load.
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Ned Simmons wrote:

Could this safety engage while the load is moving? Or just lock it once it has stopped? That's going to make a big difference in the design, particularly if it is moving at a pretty good rate.
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Paul Hovnanian snipped-for-privacy@hovnanian.com
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On Mon, 15 Mar 2010 12:09:45 -0700, "Paul Hovnanian P.E."

It will only engage if some part of the lifting gear fails (a chain was the example I gave in the original post), or when performing periodic testing.
--
Ned Simmons

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wrote:

Hey all,
Not sure where this thread emanated, and normally I don't get into discussions about elevators, but elevators have "safety devices", not safety "brakes" as I read here. Elisha Otis developed the "broken rope safety" in the late 1800's, and the principal is still used to this day on certain parts and on slow speed elevators of <101 FPM. This type safety, designated "Type A", is a toothed cam held clear by the weight of the load on the cable or chain, and which rolls freely into engagement immediately that the load is removed by any means. This action is by gravity or heavy springs starting the engagement. Stopping distance is typically from essentially "zero" to 6 inches. The only way to release after application is re-establishing lift on the cable (or chain).
Higher speed elevators use a centrifugal trip governor to activate a gradual wedge clamp, so the stop is not "instantaneous", which would cause structural damage. These are designated as "Type B". Stopping distances for "Type B" may be from as rapidly as 1 foot and upwards of 13 feet depending on method, speed, and weight. Both types of safety action is reactive against the guide rails. In Mr. Otis' case, the guide rails of the day were wooden, whereas for many many years now the guide rails are a solid steel, shaped as a vertical "T" section.
Geared and gearless elevators do have brakes at the machine of course, but these operate in a control means rather than as "safeties", although a power loss would cause full application of those brakes but not initiate "the safeties". Since my retirement 10 years ago, there has also been an industry effort to provide "cable grippers" to prevent or stop "uncommanded motion", but while presently workable these are still in a development stage, and further design/engineering is necessary to assure that these don't cause undue damage to the cables when applied.
The majority of today's hydraulic elevators have neither "safety devices" nor brakes per se, although there is some ongoing work to provide some form of overspeed safety device in the event of a catastrophic failure, either by grabbing the plunger or even the rails (as with the geared above) as long as the elevator has solid steel rails. Many hydraulics use rails formed from sheet into what is called an "Omega" section, and therefore not suitable to the crushing action of the type A & B .
There is a further "Type C" for elevators over 500 FPM and with reduced overtravels, but nothing to do with this thread any=way.
Take care.
Brian Lawson, Bothwell, Ontario.
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Brian Lawson writes:

That's a term I'll have to remember.
"The expensive bit was broken by uncommanded motion into the workholding vise."
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"Brian Lawson" wrote :

Great description of those safeties. I think the cable grippers you are referring to are these:
http://www.hollisterwhitney.com/products/gripper.html
Which are now regularly installed both in the Canada (where code required it first) the US and world-wide to prevent "unintended motion". I think these can accurately be described as "Safety brakes" because they actually use brake pads to slow the car to a stop over a controlled distance, as opposed to as fast as possible.
Interesting how terminology changes, I never heard of "uncommanded motion". I like the term "uncommanded motion".
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Stephen B.
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Ned Simmons wrote:

There's a considerable amount of difference between locking the load in place once it has come to a stop and having the lifting gear fail (cable or chain break) if this load is already descending at a pretty good clip.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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I think we are getting closer to design parameters that will work for you.
Will the safety device need to operate immediately in case of power failure, or hydraulic or air pressure? Or just if the chain fails?
If all three, then somewhere there needs to be enough energy stored, compressed springs, weights, etc. to deploy the safety. If just the chain failure, then we need to discover how the device will recognize a chain failure and not a power fail or just turning the machine off at the end of a shift.
Paul
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On Mon, 15 Mar 2010 12:49:23 -0700 (PDT), " snipped-for-privacy@coinet.com"

Only in case of a failure or a test. I think a simple linkage tied to the rod that connects to the end of the chain will do it. The plan is to spring load the rod so that it retracts when there's less than a couple hundred pouns of tension in the chain. I'm working on it now and will post a pic when it looks practical.
--
Ned Simmons

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Ned Simmons wrote:

How about a safety latch system. Real simple design. First weld a stop rack to the column. Only needs to be 1/2" thick or so. Then for the stop a simple rocker that is spring loaded. On the rocker you place two sprockets that will engage the chain. The chain running between the two sprockets holds the latch off the track. If the weight drops on the chain the weight of the latch drops it into place.
I OI I IO I
I is your chain, The Os are the sprockets. With weight on the chain the sprockets stay in this position.
I O/ / /O I
Chain fails and the sprocket and mount rock down and the latch drops into the track.
Wouldn't need to use sprockets if the space is tight. Simple nylon or UHMW plastic could be used as rubbing blocks.
Simple, Safe and easy.
--
Steve W.

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wrote:

There was something very similar in one of the patents I looked at. Unfortunately, I have about .050" between the attachment points for the chain and the column.
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Ned Simmons

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Ned Simmons wrote:

One concern that I can think of is that automotive brake pads are made to be used regularly -- how are they going to work on that one application after years of neither being used or tested? For that matter, after years of building up dust and grease?
--
Tim Wescott
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wrote:

There's also the question of break-in of the pads, though some of the pad materials claim no break-in is required.
The environment is dusty, but not greasy. In any case, periodic testing and inspection of the braking device as part of the lifting mechanism, is certainly a requirement.
--
Ned Simmons

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wrote:

A picture of the device. The purple plates are 15x22x3/4 and there's 5/8" clearance between the plates and the gray column. No clearance between the column and the yellow chain brackets.
http://www.suscom-maine.net/~nsimmons/news/VerticalCar100315a.jpg
--
Ned Simmons

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Ned Simmons wrote:

The ratchet plate I mentioned in my other post could readily fit in that 5/8" clearance you have. Indeed, you could do one on either side, double ratcheted, providing four ratchet surfaces and you could make the pawl plates engage several teeth, so you should be able to handle any possible load.
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"Pete C." wrote:

Looking further at your drawing, I'd take a close look at the two column auto lifts and see if one of those might provide a good starting point to modify for your application and save some work. These lifts are remarkably inexpensive these days, ~$3k for a basic one.
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wrote:

Thanks, Pete. I knew I had seen those ratcheting safeties, but couldn't remember where. I started looking at patents for auto lift safeties and see that some use a slotted bar, which hadn't occurred to me and looks promising.
For example: http://www.google.com/patents/about?id=HaEvAAAAEBAJ&dqC31219
Re the auto lift as a whole, the most restrictive factor in the design of this device was the available space. It also travels at a much higher speed than an auto lift and supports very asymmetrical overhung loads in some orientatiions. It was supposed to be a quick and dirty prototype, but is morphing into a permanent solution, if all the safety issues can be addressed. I wish I could say more -- the manufacturing operation is pretty wild and metal related.
--
Ned Simmons

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