Pneumatics: Choose the Best Valve for Optimum Performance

  This webinar will be available afterwards at designworldonline.com & email

  Q&A at the end of the presentation

  Hashtag for this webinar: #DWwebinar

Paul Heney Design World

Nelson Tansey Humphrey Products

Pneumatics: Choose the best valve for optimum performance

•  Every application has unique requirements and functionality. Choosing the right pneumatic valve to produce your applications optimum results involve greater attention to detail. The following considerations will lead to selecting the right valve for the application.

•  Balanced design – forces of the supplied air pressure are neutralized and as a result coil force only must overcome the force of the return spring and minor friction. This results in reduced actuation force, lower power consumption, greater flow and isolation of flow paths.

•  Unbalanced designs – in the first design (coil & plunger), the return spring is of sufficient force to seal the valve against the supplied air pressure. In the second design (electropact or diaphragm/poppet), air pressure is the primary force to return the valve to its original position, which assures that the valve will return even if the spring fails. This results in very simple construction, small size, low leakage, low cost.

•  Basic design characteristics •  Benefits of each •  Typical applications

•  Uses a rubber molded poppet wrapped around the stem that moves along the bore of the valve, creating a seal when the poppet is in its seat.

•  Advantages: o  compatibility with a variety of media other

than compressed air o  few dynamic (sliding) seals o  less lubrication o  tolerant of contaminate o  high flow capability o  faster response time due to short stroke

•  Expands on the poppet design by use of a diaphragm by using the outer webbing as a guide for the poppet into its seat without using a sliding seal.

•  Advantages: o  greater reliability o  no lubrication o  compatible with a variety of media o  no sliding seals/strong seal control o  high durability o  low leak rates

•  Designed using lapped/shear or dynamic seals. Results can lead to higher leak rates, and limited use with media other than clean air.

•  O-Ring spool design o  Advantages: Small size, high functionality, and least expensive.

Disadvantages: o-ring is mechanically squeezed to create seal and a significant amount of o-ring surface area remains in contact with the bore. Design is vulnerable to high friction issues without lube or CDA. Shorter life cycle.

•  Baked-on seal design o  Advantages: Larger in size, relatively inexpensive, medium

functionality, minimal friction, and minimized shifting forces. Disadvantages: Rubber is bonded and then ground which makes it very stiff and unforgiving. Has continual leakage which increases over time.

•  Lapped-spool design o  Advantages: Small size and high functionality, spool rides on air cushion which produces minimal friction.

Disadvantages: Requires built in continuous leakage, difficult to perform under certain mountings due to shifting difficulties, must have CDA and is very expensive.

•  Poppet design o  Advantages: Has low friction and most reliable, no sticking because stem and seal don’t slide, resistant to poor

air quality, no lubrication required and is relatively inexpensive. Disadvantages: Larger in size, low functionality, less attractive.

•  Consider the application and environmental conditions o  Explosive materials (gas, dust, ammunitions) o  Temperature ranges of the applications (ambient condition) o  Continuous duty, intermittent, latching - how often/for what extent of time will it

actuate

**Note: There are also manual/mechanical operators, however we will not be touching on those during this presentation.

•  Air-pilot valves use a remote air signal to shift the valve and internal air or spring to return it when the air signal is removed

•  Can also use a double air-pilot which uses a momentary air-pilot signal to shift the valve and a second momentary air-pilot signal to return the valve to its original position

•  Indirect-acting valves utilize a small direct-acting solenoid valve to pneumatically operate a larger air-piloted valve.

•  Require minimum air pressure ratings

•  Advantages: o  High flow capacity in smaller overall sizes o  Lower power consumption o  Greater shifting forces o  More variety

•  Direct-acting valves utilize the force generated by the magnetic field of the solenoid to operate the valve. When the electrical current is removed, a mechanical spring returns the valve to its original position.

•  Advantages: o  No minimum air pressure required o  Extremely low leak rates o  Simple construction and more robust at lower comparable cost o  Typically multipurpose o  More applicable to customization such as greater flow, lower power, and faster

response times

•  How clean is the air in your system? Is there potential for contaminate to penetrate into your air?

•  Moisture •  Dust •  Particulate

•  Is there a required rate of flow •  What response times are needed by the valve for your

operation o  Minimum/maximum

•  What is the distance from main source to area where work is being done o  Length of lines

•  Size the correct valve based on flow rate rather than actuator port size.

•  Using Cv is a quick way to approximate air flow. You cannot get 100% accuracy from this calculation because air changes shape, direction, velocity, and relative humidity at the drop of a Farenheit or Pascal.

•  T1 times are measured from point 1 (valve energization) to point 2 (10% of supply pressure detected at valve outlet port).

•  T2 times are measured from point 2 (detection of outlet pressure) to point 3 (90% of supply pressure). •  T3 times are measured from point 4 (valve de-engergization) to point 5 (10% of supply pressure exhausted from outlet

port). •  T4 times are measured from point 5 (detection of pressure drop) to point 6 (90% of supply pressure exhausted).

AC/DC Voltages Coil Voltage

T1 T2 T3 T4

DC 0.010 sec. 0.001 sec. 0.005 sec. 0.002 sec.

AC 0.010 sec. 0.001 sec. 0.018 sec. 0.002 sec

•  3 Way, 24 VDC One Air Line (1/8 inch I.D. x 36 inch long) •  100 psig air supply Air Cylinder (1.062 inch bore x 4 inch stroke) •  Volume = 0.785 x Diameter2 x stroke or length

•  Cylinder Volume = 3.54 cubic inches •  Air Line Volume = .044 cubic inches •  Total Circuit Volume = 3.98 or 4 cubic inches

•  T1 Time to energize the valve =0.010 sec •  Time to fill 4 cubic inches •  40% of 0.2 for 10 cubic inches =0.080 sec •  T3 Time to de-energize valve =0.005 sec •  Time to exhaust 4 cubic inches •  40% of .32 for 10 cubic inches =0.128 sec •  Total Cycle Time =0.223 sec

•  Volume of actuator •  Cycle time

Example

2 in bore x 6 in stroke (area x stroke) = 18.8 cu.in = .011 ft cu 1 sec. stroke time 0.65 cfm

•  What does failure mean to you in your system? •  Actuation (air, solenoid; direct v. indirect)

o  What happens to the valve with loss of electrical signal? o  What happens with loss of air supply? o  What happens with increased air supply?

•  Force to return (spring, air spring, detent) •  What direction should the valve fail…Fail open/fail closed,

fail locked (last position)

•  Type of valve – poppet, spool, diaphragm •  Air or solenoid •  Direct or indirect •  Flow rate, size •  Failure conditions

Design World Paul Heney pheney@wtwhmedia.com Phone: 440.234.4531 Twitter: @DW_Editor

Humphrey Products Nelson Tansey NTansey@Humphrey-Products.com Phone: 269.381.5500

  This webinar will be available at designworldonline.com & email

  Tweet with hashtag #DWwebinar

  Connect with   Twitter: @DesignWorld

  Facebook: facebook.com/engineeringexchange

  LinkedIn: Design World Group

  YouTube: youtube.com/designworldvideo

  Discuss this on EngineeringExchange.com