Rod locking pneumatic cylinders provide emergency locking and braking
30 June 2017
In automation applications, fail safe is a pre-requisite for any installation. In order to prevent damage to the products being assembled or manufactured, the automation system or line, or, most seriously to alleviate the risk of injury to machinery operators or those in the vicinity, any kind of pause in operation must leave the equipment in a safe state.
The most common cause of a failure in a process is an interruption in the powering of the system as opposed to the catastrophic and permanent failure of either single or multiple elements. In industrial automation, this method of power could be electricity, hydraulic fluid or compressed air. In less well maintained supply systems, the risk of failure is greater, and that applies to electric, fluid or pneumatic power. In the case of electricity, more sophisticated applications or those where the consequence of pauses in the supply of power would be either dangerous or expensive – for example medical or heavy industrial, then uninterruptable power supplies and generators can bridge the gap during power outages; the same is not possible with hydraulic or pneumatic systems.
Hydraulic and pneumatic systems are often used in scenarios where they are required to move loads from A to B or simply hold them for a period of time. For the purpose of this article we will focus on pneumatics, where removal of power is typically caused by either a break or significant drop in the pressure of the air supply. This may be caused by either the failure of a compressed air pump or other equipment in the line, or by a major breach or leakage in the supply line to point of use.
So called rod locking cylinders have been introduced by companies like Parker Hannifin for use in pneumatic applications where safety is critical and failure in a safe state is a necessity. Typical end applications include materials handling systems and industrial or production position control.
Pneumatic cylinders such as Parker’s P1D Series of ISO cylinders have been designed to hold loads safely in position in a situation where the compressed air supply fails. The rod locking feature is provided by the cartridge through which the piston rod passes and is mounted transversally in the rod lock unit of the cylinder. When a failure occurs – represented by a loss of ‘signal’ pressure – the force from a set of internal springs pushes the mechanism to lock the piston using jaw clamps. Cylinders like Parker’s P1D-H can only be used in dry piston rod applications as any grease, lubricants or coolants will reduce the effectiveness of locking.
Rod locking cylinders are available in two basic variants – one for dynamic applications, Parker’s P1D-L variant, the other for static designs, Parker’s P1D-H variant. Both are able to hold the piston at any point in its stroke. The locking mechanism disengages when air pressure is re-applied so that normal operation can resume. Importantly, rod locking cylinders should not be used as the only safety component in an application – they are only designed for emergency use and other technologies should be used to achieve planned braking or holding of the system. Parker’s rod locking cylinders use air/spring activation with the mechanism integrated into the cylinder design to save space and simplify and speed installation.
Certain rod locking pneumatic cylinders may be used for dynamic applications, that is, they can, within stated limits, be engaged when the rod is moving as a brake.
Parker’s P1D-L is an example of a dynamic rod locking cylinder, it is based on the axial movement of a piston. With no signal pressure, a number of high force springs push the piston to move axially. This movement is applied onto the system of the conically shaped inner ring of the piston and translated into a radial movement of a number of clamping sleeves via hundreds of high precision steel balls. The result is that the hardened steel clamping sleeves are pressed onto the piston rod with a high and stable force.
The surface of the clamping sleeves applied to the piston rod have a special design with a pattern of grooves to provide maximum grip. This design feature improves the holding capabilities of the cylinder significantly – especially in applications where there is a risk that the piston rod will be exposed to oil, grease, dirt or other liquids and contaminants.
In static applications – where piston rod locking only takes place when there is no movement of the piston and there is a no pressure ‘signal’ - the lock rod unit as with Parker’s P1D-H series, is typically external to the main pneumatic cylinder and is flange mounted onto the front end cover of the cylinder body.
The locking force is provided by the cartridge which is mounted transversally in the rod lock unit. The piston rod passes through the cartridge and when there is no signal pressure, the force from the internal springs pushes the over-the-centre mechanism to lock the piston with the jaws clamping onto the piston rod and holding the cylinder firmly and safely in position.
In the case of the static rod locking cylinder, the sharp edge of the jaws will be damaged if the piston rod is not stationary before the rod lock unit is engaged.
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