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.
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.
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.
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
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.
Great description of those safeties. I think the cable grippers you
are referring to are these:
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".
Remove the first Spam only to e-mail directly
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
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
I think we are getting closer to design parameters that will work for
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.
On Mon, 15 Mar 2010 12:49:23 -0700 (PDT), " email@example.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.
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
I is your chain, The Os are the sprockets. With weight on the chain the
sprockets stay in this position.
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.
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?
Control system and signal processing consulting
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.
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
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.
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.
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.
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