A thorough machine risk assessment should focus, at least initially, on the actuators, whether they are cylinders, air motors, air bags, or clutch brake mechanisms. We suggest starting here because safety systems generally de-energize the directional valves that control the actuators when a safety event occurs so it is crucial to consider the outcome when these valves become de-energized. What type and magnitude of hazard(s) could occur if a valve malfunctions when being de-energized? Take time to consider these potential scenarios during your risk assessment to help prevent future machine damage, downtime and personal injury.
Even though most pneumatic systems operate in the 80-100 psi range, there are many variations to this, both higher and lower. To make our point, we will concentrate on this common pressure range. However, you should consider your actual system pressure when doing a risk assessment on your own equipment.
The pressure supplied to an actuator is literally the driving force and the higher the pressure, the higher the force the actuator can apply to the work piece (and to an associated pinch point, etc.). The chart below indicates how much force can be generated by standard size cylinders at both 80 psi and 100 psi.
According to ISO standards and some pending ANSI standards for pneumatic cylinder presses, you must take additional safety measures to protect against personal injury and/or machine damage for any force greater than 150 N/33.7 lbf or a load greater than 15 kg/33 lb of mass (including tooling and load). A single-channel valve is an unacceptable solution because any malfunction in the valve that allows pressure to continue to flow downstream could result in pinch-point forces exceeding these standards allowances due to a fully pressurized cylinder or a cylinder with a high gravitational load. Thus, redundancy and monitoring are required which raises your minimum control category to at least 3.
It is easy to see that forces higher than these are attainable in most typical machine operations depending on actuator size and supplied pressure. In cases where these force threshold values are exceeded, additional measures must be utilized in order to provide adequate machine safeguarding. Other factors, such as tooling, may result in much lower force threshold values being used in the risk assessment. You should examine these factors as well because they may make a difference in the cylinder’s potential for damage. For instance, a blunt ended cylinder with lower than 150 N or 15 kg force poses fewer risks than a cylinder with a guillotine or piercing tool attached to it operating at the same low force. Likewise, a cylinder stuck in the extended position would not have the same risks as a cylinder with a heating element (in a heat seal application) stuck in the engaged melting mode.
How can you avoid some of these hazardous conditions? There are three primary solutions for abating pneumatic actuator associated risks:
1. Reverse the cylinder motion to a safe position by using a 5/2 control-reliable device such as the ROSS CROSSMIRROR double valve. This will result in a safe state provided the return stroke of the actuator does not pose any additional risk. In the case of the heat sealing application, this could a very good solution. It would remove the heating element from the work piece as well as reverse the cylinder direction away from the pinch point.
2. Remove the air supply to the control valve and, therefore, to the actuators with control-reliability (Category-3 or -4). Use a safety-rated valve such as a ROSS’ DM2 series valve matched to the control category determined by your risk assessment. The advantage of this method is that one safety-rated exhaust valve can be used to remove the supply pressure from one or more directional control valves and actuators while helping maintain safety system control integrity.
3. In some cases, removing the supply pressure alone can leave a load free to fall or continue moving due to gravity or momentum. In addition to solution 2 above, consider the gravitational force and momentum operating on the mass of the load and apply a suitable solution to stop and hold the load in place. With the motive force removed, the suitable solution will be dependent upon the mass, the tooling, and the failure modes of the devices being used to maintain a safe state. Solutions may include the use of pilot-operated check valves to trap pressure beneficially or safety catchers/rod locks to mechanically hold the actuator(s) without trapping pressure in the system.
Shmuel Rabinovich says
Hi,
Please let me know from where the table is ?
and the standard that according to what standard is the value 150 N
Thanks
Shmuel