Safety Brake for Spinning Shaft?

There are several systems that I've seen or used. The big question is how much inertia the shaft has. That will determine how long it takes to stop.

Here's one of the slickest ones I know of. It's only for low inertia systems, but I've seen it demonstrated and it stops right now.

http://www.sawstop.com/how-it-works-faqs.htm

And how much torque your motor can take. Are you just spinning up the shaft? A fairly loose belt drive might get you to 6000 rpm and since the torque capability is low and as TP45 points out if the inertia is kept low, then you should be able to grab it with your hand safely and bring it to a stop.

Here is a rotor kit from Bently that they use to demo rotordynamics. We used to (in a more reckless day) use a hand on the wheels to stop these...

http://www.ge-energy.com/prod_serv/products/oc/en/downloads/141592.pdf

From what I hear, it seems the best advice would be to put a second (clear) guard bolted over the moving parts + the interlock. That way, by the time you un-bolted the second guard, the interlock would have broken the circuit long enough for the device to spin down.

A few risk assessment question:

• how often does someone actually go into the enclosure, ?
• how accessible is the key?
• what is the consequences?
• would another bolted (clear) guard provide enough protection?
• are there other options?

I have worked on many systems that use electric brakes for quick stopping. These devices with no power, have a very strong brake applied, When you energize it, it releases the brake. It would be in connected to an interlock or switch, and physically attached to the drive shaft. Any time the power is lost or the interlock opens the cct, the brake is applied (usually with very powerful springs)

Here is a link to some suppliers.

An interlock is absolutely necessary, simply a switch or relay in series with the power that opens the cct when the door is opened. The second guard is highly recommended. It will also protect if access is gained by other then the interlocked door. The electric brake is dependant on your risk assessment.

There is also the option of having a motor which slows itself down when it loses power by reversing the field such as are used in some table saws. These do take a few seconds in slowing down though.

As far as a standard no. As you have recognized that you have a safety problem OHSA would have you correct it. The enclosure could be all that is required. It is locked and secured. Your worry is some idiot entering the enclosure while the shaft is spinning. Isn't the locked enclosure to keep idiots out and away from moving parts. Its is not your fault that someone gave an idiot a key.

Still if we must protect all the idiots of the world. A guard over the spinning shaft. A warning sign requesting that all personnel that would enter wait until all moving parts come to rest before the do so. The sign would be a good ideal anyway. The lack of knowledge of the hazards do not make one an idiot.

In machine design, this is something you need to determine following a set formula that takes you through several options all depending on the severity of injury it can cause at what stage.

Once you have decided that it is possible, given the time delay and operations possible by stupid operator, that somebody gets injured, you need to prevent this.

Prevention is not just telling you to stop the motion that can kill or injure, it tells you that contact causing the injury should be prevented. This means you can easily fit a time delay, calibrated to the time it takes to spin down, before it allows you to open the hatch or door.

Look at interlock devices from Pilz or Guardmaster. I am sure there must be way more available but these stand out in my mind for good design, reliability and enormous range in their product catalogues.

I'd suggest a solenoid release locking guard switch with a Stopped Motion Detector to release.

Links are for Guardmaster/Allen-Bradley items. Other manufacturers can supply similar items.

Put a padlock with only one key to fit it on the access opening. If you or a trusted relief person has the key control is established and third parties are denied entry and access to mischief!

This is a standard industrial procedure for many varied applications.

If somebody gets injured this way, it will never wash with the courts as you did nothing to decrease the chance or the possibility of it happening.

Sorry but your solution is unacceptable in dangerous machine design, back to the drawing board I am afraid.

Any possibility that you could fit a rotor to the shaft for a brake caliper to act on? This set up is used on Clausing Colchester lathe spindles. When energized the brakes release. When any safety device is tripped the motor shuts off and the brake stops the spindle quite quickly. I think the brake caliper is adjustable via spring pressure. I have not had to adjust these yet so I am not entirely sure.

Many people seem to think that a brake is the only or the best option.

Normally the application of a brake depends on the number of entry points and the frequency or likelihood of it happening. If there are many entry points to the point of danger, such as in a printing machine, it becomes too difficult to guard all the entry points successfully so a brake is applied ONLY to bring down the time of exposure to the danger. The brake DOES NOT take the danger away, that is done by the guarding and the time calculations for entry, even when forced.

Example: if you use a light curtain for a frequently entered area, you need to take the worst case scenario for the time it can take from breaking the curtain to the point of danger. This is normally a person running who than trips and falls through the curtain. Your machine needs to stop in THAT time otherwise the person can get injured by the moving parts of that machine. (I know he or she will likely get injured anyway but not by the machines moving parts)

If a brake gets the time for the machine down to the acceptable frame, you are required to fit one. If it does not do this, you must redesign the entry point and or guarding.

A brake is expensive for various reasons so it is not the favorite choice. They need maintenance and will increase the maintenance need for the machine as they are not nice to moving parts.

The rule is simple, if you can fit a prevention of entering that bridges the time you need for a fraction of the price than that is the option to go for. Especially if there is only ONE point of entry the whole thing seems easy and clear cut to me. Count your pennies and go for a beer with the money you save

just a couple of other most vsd,s have a brake/ ramp down function in them if inertia isnt a problem you can use the vsd as a brake eg dorr opened vsd ramps down in .5 second. as other posts have suggested there are many interlocks that monitor a shaft/pulley etc and through an interlock door doesnt open until the shaft has come to a complete stop.

"Does anybody know where I can find a reference to a standard that addresses this issue?"

Isolating the Risk of Catastrophic Flash Explosions
Reference is made to OSHA, and NFPA. OSHA would be most likely to cover your needs.

Safe Use of Mechanical Equipment - Univ. Edinburgh Presents details for safely protecting of individual from various classes of equipment hazards.

Despite the many recommendations made it this thread NO ONE including the self-appointed 'experts' specifically answered your main question of concern, the matter of the governing authority on the matter at hand.

The problem with that is that we all live in geographically different areas where different laws and different governing departments rule.

I used to work according the CE self certification guide of compliance which is done according the machine directive 1 to 4. USA will be totally different as , importers of machinery will know, it is very difficult to get a machine that complies to local rule when you import it from far away. It took us many years of "discussions" to get the appropriate technical documents for accreditation. In the end we enforced a little known rule that states that the importer can do their own accreditation if they do the installation and warranty issues as well.

For local knowledge of this kind we need a local "so called" expert.

You seem to be using a mechanical brake, I would only have that as a secondary system. I would suggest that Electrodynamic braking should be used as a primary system to stop the shaft.

Basically this consists of a suitable relay that normally applies power to the motor, that when it is de-energised, it causes either a short across the motor power connector or via a suitably dimensioned low value resistance.

This is the fastest and safest way to stop such a shaft. It will usually stop it in the same (or close to) time that the shaft takes to speed up from stop in the first place.....if that would be too slow, then use a mechanical brake as well.....

Using dynamic braking as a first option has the advantage of adding little extra wear and tear provided the whole design was mechanically sound in the first place.

It works on both DC & AC motors (single or 3 phase) of any type that I am aware of personally.

This post assumes that you are using an electric motor directly to spin the shaft....you did not specify.