Machine Guarding....

[jim01018]

Hi

I have a question about machine safety and the dreaded gaurding.

From Brish Standards any hole over 40mm that leads to a piece of moving machinery needs to be guarded by a guard sufficient to keep the person away from the danger area. As in the case of a hole for example a tube of 850mm in length can be fitted to keep people putting there arm down and touching moving parts.

We have an application whereby this length of guard is no good in production and is not fit for purpose. There is no other type of guarding we can use (i.e swan neck type etc) as the product needs to 'feed' in a straight line into the machine (extruded products for example).

I have read that adjustable guards can be used whereby the operator can physically undo the retaining screw and slide the guards away to suit different size products, is it possible to have this type of guarding with proper detailing of instruction manuals and risk assemssments, and as a company are we covered ? I mean , should the operator not slide the guards back in place after use, there is nothing still stopping them putting their arm down into the machine.

The guides from British Standards and the HSE are somewhat vague and non of this has really been covered.

Can any one offer any advice on this subject ? It would be greatly apprecitated. As a company we obviously want to supply machines that are safe to use, however they also need to be practical.

Thanks anyone and all

 

[MJCronin]

jimbo...

Here in the US, the definitive guide to guards for rotating machinery is ANSI Z535.1-1990. It has been referenced in the US OSHA guidelines and regulations.

A website you may want to visit is:

http://www.osha.gov/SLTC/machineguarding/standards.html

If you GOOGLE guards and OSHA, you can get invovled in a bazillon little interpretations, by OSHA of the standards.

Your question about an adjustable guard may be discussed there.

My opinion only

-MJC

 

[DesignerGuy16]

I'd think your guards should be sized so that in your worst case scenario you cannot reach the danger zone of the machine.

There's minimum distances from danger zones defined by how large the opening is at the end. Guards can be "fixed" with screws, but you might still need to add a safety switch to confirm the guard is in position to allow operation. Extruded product or not at different lengths, figure out your max guarding opening and make it's length based on what the standards say you need to be so that someone cannot reach a "danger" zone while the machine is in operation. Things like lightscreens work great for this, and they're pretty smart to know the difference between an extruded product/box and an arm.

I'm not familiar with British Standards, but this goes for most all safety standards I've worked with.

James Spisich
Design Engineer, CSWP

 

[ctopher]

I used to designed machinery almost 20 years ago. I added thick Lexan as guards bolted in place for future removal. In addition, all emergency stop buttons were immediately behind the operator's hand.

Chris
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[KENAT]

If you have a movable guard can you not have an interlock that prevents the tools operation if the guard is not in place? I think this is how we've handled similar situations.

We have used light screens in another similar place.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[HEC]

Jim,
An alternative method would be to use the feed tube to "guard" the entry and a detector to ensure there is metal in the work zone, not flesh! This is only a basic idea and requires much fleshing out!

Mark Hutton

 

[jim01018]

Thanks people for your help,

We do use movable guards (of a sort) which utilise coded magnets.

The main problem is that if a customer wants to cut something really short, yet the diameter is over 40Ø, to use a long guard (850mm +) it is just not practical. If we did supply a machine of this sort then there is a chance the customer would remove the guard. This is not acceptable at all.

Why i asked about the risk assemssments and instructions and the use of 'Adjustable Guards' is how do milling / drilling machine manufactures get around this problem ? They often have adjustable guards on the machines to cover the drill bit and protect the operator. Depending on the length of drill bit etc these type of guards are usually adjustable. However i have known of people just not even using them cos its quicker. If this was the case and the operator got cut by flying swarf etc what protection has the milling / drilling machine manufacturer got against proving / stating they have supplied a safe machine ? Would a detailed risk assessent and instructions on 'You must use the guard at all times' etc etc be sufficient ?

Problem with guarding is a little quick peek tends to open up a can of worms ! I always try to design our machines intially around keeping safe distances, if this is not acceptable due to production requirements then we must provide an alternative.

Tricky to say the least.

 

[DesignerGuy16]

Well in our day and age of "we must protect the operator from being an idiot" instead of not having idiot operators, it tends to make the operators lazy and they'll circumvent just about anything you can come up with. Why? Who knows, but I've heard some real horror stories of people dodging robotic safety circuits and having some bad things happen.

A detailed DFMEA should be a requirement as it is permissible in court to show that you've taken sufficient action to safeguard your equipment from really any failure modes and hazards. Just going through this motion you'll probably come away with more ideas on how to make your equipment safer.

James Spisich
Design Engineer, CSWP

 

[BrianPetersen]

Manual lathes, milling machines, and drill presses are not provided with guarding (of the tooling point-of-operation and rotating tooling or workpiece) that is in accordance with those standards, because you CAN'T.

At least in this country (Canada) there is not any *legal* way to deal with this type of situation that still allows the machine to be used.

The reference to BSI standards makes me think that you are Europe, in which case you should look up the ECE standard for the specific type of machine that you are producing; this standard may give some help of the expectations. Or maybe not. The people who wrote the standards are under the gun from the legal and enforcement side to make the machine idiot-proof but at the same time they may (or not) recognize that it can't be done.

 

[KENAT]

As we used to say back in the UK in defence:

"You can't make it idiot proof, they just come up with a bigger idiot"

Legally, I believe you usually have to have done 'due diligence' to use a bit of an Americanism. Trouble is knowing what the court will accept as 'due diligence'.


KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[GregLocock]

Hmm, I've found that if you apply money cubed you can get the risk down far beyond what people who aren't losing limbs would consider acceptable. Exactly what is it in your process that /demands/ that squishy bits and moving tooling or moving parts need to be in proximity? Ever? I bet it is laziness on your part or dollars.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jim01018]

Greg..its not exactly laziness because im actively trying to find a solution. Sure i can throw money at it, make our machines more expensive, become uncompetitive, slowly loose business then become redundant but thats not really something id like to do.

Whatever you do to guard machines there will always be people who are idoits and will try to cut corners. Personally, i try to ensure that they a) cant and b) dont need to.

If deisgn a machine with a guard to fixed standards and the customer cannot use that machine due to production requirements (i.e the product to be cut is so short it will just sit in the guard and cause a jam) then they will go elsewhere to another company who perhaps arent so proactive. Once again, we loose business...

 

[MechEng2005]

I agree with those that suggested a switch indicating the guard is in place prior to allowing the machine to run.

Another option would require the operator to hold down two push buttons for the machine to run so that you know where their hands are. There are requirements for spacing, height, etc you'd have to look into.

Obviously, both solutions are able to be circumvented, and what KENAT said about "due diligence" is spot on. Unfortunately, engineers designing the machines are (apparently) not qualified to judge what is safe (not that all engineers would agree anyways).

-- MechEng2005

 

[GregLocock]

jim, those are the arguments (well, excuses) that I have always heard.

I repeat:

Exactly what is it in your process that /demands/ that squishy bits and moving tooling or moving parts need to be in proximity? Ever?

You imply that cost is the main barrier. I'm not quite sure why people get so complacent about a moment's inattention costing the operator a limb.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[DesignerGuy16]

If the machine needs to be that flexible, I'd recommend looking at a light curtain with the 2 palm button operation.

With the light curtain you just need to space it far enough away that if you break it there's enough time to stop the machine before the operator can physically touch it. There should be calculations for this, I've used similar when I was working in robotics.

The 2 palm buttons assures the operator is out of reach and the light curtain will allow only what you program into it, they're smart enough to know the difference between an arm and a box or piece of material if you set it up correctly. Compared to other options it's actually pretty reasonable cost wise.

James Spisich
Design Engineer, CSWP

 

[JakeAdkins]

I agree with the light curtain and the 2 palm buttons. I have used that in the past when I had to retro fit some old equipment. It worked out very well.

You may also want to consider a ‘work piece extender’ kit of some sort. If your work piece is too short to easily work with the 850mm tube or what ever. Provide a fixturing kit that will allow the operator to safely work with the guides.

Would a kill switch like on a lawn mower work. I am not sure about England but in America our lawnmowers have a spring bar that has to be held down for the motor to run. Once you let go of the bar the motor stops.

I like how you say ‘redundant’ it sounds better than ‘laid off’

 

[DesignerGuy16]

But laid off is better than "arm off". =D

James Spisich
Design Engineer, CSWP

 

[HEC]

jim,

if the concern is for needing to hold a short work piece, then how about supplying a mandrel to hold the short pieces. Though there was the guy who lost some fingers to a tube flaring m/c, he was issued with a pair of multi grips to hold the tube, he had broke several pairs getting them caught, so rather than asking for yet another pair he held the tube in his hand... the rest is sad history. So a work piece holder will not always be used, but is a reasonable way around the problem.


Mark Hutton