Machine Safety: Pausing Index Table?

[Milt]

I used to work for a company that builds hydraulic powered rotary transfer machines for machining parts. If the machine is in cycle, an individual unit is in cycle, the drum is indexing, or the machine is sitting idle with the spindles and/or the cutting fluids on, opening a guard door is the same as hitting the e-stop button. With the machine completely idle there is a key switch that must be turned to prevent the hydraulics from shutting off when a door is opened.
Even though opening a door at the wrong time results in all of the parts in the machine being scrapped, this gives a level of safety that should not be compromised.

 

[Mylo]

Hi Milt,

Note that your mail stems from America,,


Your Keyswitch would effectifly "Bypass" the safety" Circuit and as such would allow dangerous actions to occur unless their is a method of monitoring the bypass. This practice would not be allowed under any circumstances over here. However a common approach used which may be what you are talking about is a secondary safety circuit that "Mutes" the first, this is acceptable but what if you keyswitch was left in the lock, would this allow operation of the machine??

 

[panic mode]

Have you ever seen Hydromate? This is a workhorse of high
volume manufacturing. It is just a turntable with number
of stations etc.
They don't drop the parts when cycle is stopped.

(just creating confusion ... )

 

[jshiepe]

Chako,

Just out of curiosity, is the control system the only element of the machine that is fair game for redesign? If not, maybe you could consider making some of the critical functions of the machine mechanical(i.e. tied to a main shaft and operated by a cam). I can testify that after working with rotary filling machines for over 15 years that air actuated machines tend to be a lot less reliable than mechanical ones. You may spend a little more maintaing them, but in the long run, the increased productivity pays for the maintenance many times over.

I know that there are differences between a filling machine and a parts machine, but the concept is the same. With your critical functions tied in mechanically, you would not have to worry about where the function was in the cycle, and also gain the advantage of using machine degrees from a 0-359 degree encoder to program all the other elements of the machine.

Just a thought.

Jeff

 

[Milt]

MYLO- With the switch in the open door postion no machine functions can be initiated. The purpose is to allow inspection of tools and parts with out having to completely shut down the machine. When changing tools or performing maintenance the standard practice is to lock out all power except for internal machine lighting, which is controlled seperately.

PANIC MODE- The shop I work in now has 14 Hydromats. Anytime an unplanned interruption of cycle happens, lid being lifted, spindle overload tripped, various sensors generating a fault, etc. we scrap all of the parts in the machine.

 

[chriswarrn]

I've seen machine tools that have an electrically actuated door lock mechanisms that will only allow access after the cycle stops. Of course, these were always bypassed by order of management, but it at least gets the machine tool builder off the hook.

 

[ndzied1]

but it at least gets the machine tool builder off the hook.

Perhaps in the long run but in our law suit happy USA, if an accident occurs, everying with legs will be litigated against. Especially those entities perceived to have the most loot.

Of course, no machine is 100% safe. And there seems to be an inverse relationship between safety and the ability to setup/maintain a machine. One customer's spec was written such that it was impossible to set up the machine because all power had to be off when the door opened. As a compromise, we put a key switch INSIDE the electrical enclosure. Turning this switch would allow pressure to air circuits critical to setup and allow their manual functions. It would also DISABLE automatic functions on the machine. I advocated for a key coded differently from the manufacturer's standard key but was voted down. Well, you win some and lose some.

I have worked for both kinds of customers. Those who will defeat safety items and those who will discipline employees who do. I sleep better with the second kind but from discussion here it seems that is a rare breed.

 

[Unregistered]

dumping air on indexing table

Our company designs our safety circuits to category 3, but we have found
that there are certain judgement calls that can be made regarding where air has to be dumped and not dumped.
We have 2 pneumatic circuits, each one plainly identified by color (all tubing
actuators, valves, etc.). One circuit is pressurized when control is turned on,
the other is interlocked with safety circuits.
Each motion/actuator inside the machine is evaluated for the hazard it represents (pinchpoint, stored energy, etc.) and assigned to one of the 2 pneumatic circuits. For our company the grippers can almost always stay pressurized when the doors are open because we specialize in small parts assembly and design in such a way as to minimize the hazard of a pinchpoint.
As for pick-and-places or vertical actuators dropping when doors are opened, we evaluate whether damage will occur as a result or if the part might be dropped as a result of the jarring motion. If that is the case we use a pilot operated check valve to hold the axis up but still dump all other air.

We definitely believe safety is the main concern but instead of looking at everything inside the envelop of the guarding as one entity we evaluate each motion separately for cause and effect.

 

[OkiePC]

Regarding pneumatics and safety. Each axis must be evaluated independently to determine which is safer (dumping or not dumping). Often, it's safer to leave air applied using double solenoid two position valves. (suspended loads; gripped parts) If there is a pinch point involved and gravity is not a factor we use 3 position open center valves. That way when power is removed, the axis can be moved freely by hand in either direction.