Vw0811 Actuation Oxler
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MANUFACTURER’S VOICE www.valve-world.net November 2008 77 Further to one of my previous discussions regarding pneumatic part-turn actuators, in this issue I would like to take the opportunity to report on the best choice between the different technical solutions offered by pneumatic part-turn actuators. Basically the so called scotch yoke actuator delivers the transmission from a linear movement into a rotary action, known as part-turn action, by means of a lever arrangement (see figure 1). No gear shaft, no gear rack on the piston. This makes the machining independent from any material to be used for the shaft – just think of the difference in machining stainless steel teeth and/or aluminium teeth on a shaft? Difficult if we look at hobbing and a pressure balanced shaft – so called floating shaft (see figure 2) – which makes the shaft blow out safely and reduces the wear & tear tremendously. The result is a wider material selection for the shaft as only turning & drilling is necessary and a perfect solution with low wear and tear. All the used materials can be selected according to the operating conditions and can be hardened – just If we talk about wear & tear we should not underestimate the abrasion within the gear transmission. The aluminium particles, together with the oil / grease inside the actuator, form a grinding paste which results in a even faster damage of the actuator and, in the worst case, as well the abrasives infiltrate the compressed air system and damage upstream/ downstream components such as solenoid valves; valve terminals etc. Once again we have to consider that when using pressure dependant sealing rings and/or slide rings with graphite/Teflon compound material a lubrication of the air is no longer necessary so that the effect of having abrasives inside the actuator is less destructive as well as a reduction in the maintenance (refilling & draining of oil). The lever arrangement is responsible for the different output torque compared to a rack-and-pinion actuator (linear, constant torque curve) (see figure 4). Figure.3 Figure.2 By Günter Öxler Scotch Yoke or Rack-and-Pinion pneumatic part-turn actuator? Figure.1 think on the cam disk in combination with the bearing pin in the piston (see figure 3). Frequently those actuators will reach a lifecycle time of millions of cycles. My experience shows an application with 50 Mio cycles within 6 months (glass bottle production line). The diagram shows the exact difference in effective output torque in both actuator systems. It is important to note that the scotch yoke actuator delivers a 20% higher break-away torque from the zero position – where any soft seated valve requires the highest available torque
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Actuators
Transcript of Vw0811 Actuation Oxler
MANUFACTURER’S VOICE
www.valve-world.net November 2008 77
Further to one of my previousdiscussions regardingpneumatic part-turn actuators,in this issue I would like totake the opportunity to reporton the best choice between thedifferent technical solutionsoffered by pneumatic part-turnactuators.
Basically the so called scotch yokeactuator delivers the transmission from alinear movement into a rotary action,known as part-turn action, by means of alever arrangement (see figure 1).No gear shaft, no gear rack on the piston.This makes the machining independent
from any material to be used for the shaft– just think of the difference in machiningstainless steel teeth and/or aluminiumteeth on a shaft? Difficult if we look athobbing and a pressure balanced shaft –so called floating shaft (see figure 2) –which makes the shaft blow out safely andreduces the wear & tear tremendously.The result is a wider material selection forthe shaft as only turning & drilling isnecessary and a perfect solution with lowwear and tear. All the used materials canbe selected according to the operatingconditions and can be hardened – just
If we talk about wear & tear we shouldnot underestimate the abrasion within thegear transmission. The aluminium particles,together with the oil / grease inside theactuator, form a grinding paste whichresults in a even faster damage of theactuator and, in the worst case, as wellthe abrasives infiltrate the compressed airsystem and damage upstream/downstream components such assolenoid valves; valve terminals etc.Once again we have to consider thatwhen using pressure dependant sealingrings and/or slide rings withgraphite/Teflon compound material alubrication of the air is no longernecessary so that the effect of havingabrasives inside the actuator is lessdestructive as well as a reduction in themaintenance (refilling & draining of oil).The lever arrangement is responsible forthe different output torque compared toa rack-and-pinion actuator (linear,constant torque curve) (see figure 4).
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By Günter Öxler
Scotch Yoke orRack-and-Pinion pneumaticpart-turn actuator?
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think on the cam disk in combination withthe bearing pin in the piston (see figure 3). Frequently those actuators will reach alifecycle time of millions of cycles. Myexperience shows an application with 50Mio cycles within 6 months (glass bottleproduction line).
The diagram shows the exact differencein effective output torque in bothactuator systems. It is important to notethat the scotch yoke actuator delivers a20% higher break-away torque from thezero position – where any soft seatedvalve requires the highest available torque
MANUFACTURER’S VOICE
www.valve-world.net 78 November 2008
There are also, of course, limitations tothe scotch-yoke system.If it is necessary to operate 3-way or 4-way valves which requires a turning angleof 180° it is not possible to use a scotchyoke actuator as the lever arrangementdoes not allow turning > 95º.However, considering all the benefits of aScotch-Yoke actuator, it is the best choice inautomating valves with a 90º turning anglesuch as butterfly, ball and plug valves forON-OFF as well as for control applications.
(e.g. Butterfly Valves) whereas, along therotary movement, the available torquegoes down to a value 45% lower after50% travel and increases at the endslightly to 75% of the starting torquewhich benefits the torque requirement ofsoft seated ball valves.The benefit for the user is either a smalleractuator or a higher break away torqueconsidering the same diameter of thepiston in the actuator – resulting in costbenefit and space saving.
Why two different systems?Most of the suppliers will define the twodifferent actuator systems in a way thatuses rack-and-pinion actuator forCONTROL purposes – because of itslinearity, and the scotch-yoke actuator forON-OFF applications as the scotch-yokeactuator is unable to hold intermediatepositions. If we look on a common torquecurve of a butterfly or ball valve, (as seenin figure 5), we will see that these valveshave a reduced torque demand duringthe travel – resulting from the balancedtendency to open or close initiated by themedia stream and the circumstance that75% of the torque requirement in a softseated valve is generated by the seat.But please do not forget about thepreviously mentioned design criteria: “NOO-Ring sealing as a dynamic sealing”. Thebest option is a pressure dependant lipsealing as it delivers the least possibleleakage. Second best is a combination ofan O-Ring with a graphite/PTFE slide ringwhich reduces the friction in a slip-stickeffect (see figure 6). This gives the bestpossible result for smooth operation andprecise use of valve-controllers.
Günter Öxler has along history withinthe valve industry.He graduated inProcess Engineeringand MechanicalEngineering in
Stuttgart, Germany, holds a MBAdegree in VWA as well as a Controllingdegree and is a REFA Specialist. Formore than 25 years, Günter Öxler hasworked for several companies in thevalve business, companies such as J.M.Voith GmbH (Hydropower and PaperMachinery), Erhard GmbH (R+DProcess Valves and project engineering),and Festo AG & Co. KG (ProjectManager and Project Engineer ProcessAutomation). He is also member of theIWA, ISA, and VDI German Engineerand he is multilingual as he speaks 5languages, among which are German,English, French, Italian and Spanish.Günter can be contacted under:[email protected]
Meet Günter Öxler
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