Machine Safety: Actuator controls versus machine actuators

Why is knowing the difference between a machine actuator versus an actuator control important? The hazard on a machine is what’s really important to reducing risk! Understanding two standards-based definitions can help.

06/12/2013


Why is knowing the difference between a machine actuator versus an actuator control important? The hazard on a machine is what’s really important to reducing risk! 

What is the hazard? Motion caused by a machine actuator can be a hazard to an operator, maintenance technician, set-up engineer, or clean-up personnel, to mention a few. However, the power (electrical, hydraulic, pneumatic, etc.) applied to the actuator control also can be a hazard to the same personnel. So, is looking at the hazard the best way to understand these terms? 

Perhaps some definitions of these two terms may help clear up the confusion. Several international and domestic standards define:

  1. An actuator control as an operator control device used to initiate or maintain machine motion or other machine function(s). Some examples given include; two-hand control, treadle bar, foot control, inching button, e-stop device, etc.
  2. A machine actuator as a power mechanism used to cause motion of a machine. Some examples given include; air cylinder, motor starter, hydraulic valve, etc.

That said (and not to add more confusion) the machine safety standards also explain that a PSD (presence sensing device) device is an example of an actuator control. An example is when the robot arm retracts from a light curtain field initiating a signal to the machine control system to automatically cycle the machine. Or, when a “safe drive” automatically detects a fault and causes a Cat 1 machine stop. Where’s the operator in these examples? 

Regarding machine actuators, I’ve also seen the metal rod inside an operator control device described as an actuator because it causes the contact(s) to open and remove power. Therefore, can a machine actuator also cause the stopping of a machine motion as well as causing machine motion? What about the friction clutch in a mechanical power press? Is this device a machine actuator since it either causes or stops motion and is powered via electric, hydraulic, pneumatic or spring type energy? 

I believe by now the answer might be as clear as mud. So, what’s the point? It’s my opinion that this discussion is a good example where industry experts can help interpret the intent and requirements of machine safety industry standards. After all, has anyone ever seen a prescriptive Standard for compliance?

Has this presented you with any new perspectives? Add your comments or thoughts to the discussion by submitting your ideas, experiences, and challenges in the comments section below. 

J.B. Titus, CFSE

Related articles:

Inside Machines: Does adopting ISO 13849-1:2006 change the U.S. model for compliance and enforcement?

Machine Safety – does OSHA reference consensus standards for compliance?

Machine Safety: Is OSHA okay with my 'acceptable' risk mitigation?

Contact: http://www.jbtitus.com for “Solutions for Machine Safety”.



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