Linear Actuators

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Linear Actuators
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Typical Construction
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Common Fab Methods for Off-Road Applications

• Tie Bolt Construction

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Common Fab Methods for Off-Road Applications

• Welded body
• Nonrepairable
• Repairable

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Basic Types

• Single acting
• Double Acting

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Basic Types

• Double Rod Design

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Basic Types

• Telescoping Designs
• Short collapsed length
• Long total extension
• Force capacity declines with stages
• Usually single acting

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Mounting Methods

• Clevis
• Face flange
• Lugs on the end caps base or centerline
• Trunnion
• End
• Midline

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Cylinder Construction with Seal Locations
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SealsRod Seals

• Rod Seal (one or two)
• Seals in fluid
• Can use a drain to port leakage to tank
• Simplest is an O-ring with backup
• More often a U or V Lip seal

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SealsRod Seals

• Wiper seal
• Must keep foreign matter out
• Note Often use a cartridge with both rod and
  wiper

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SealsPiston Seals

• Common types
• O-ring with backup ring Single or Double
• U cup or V cup seals with or w/o Backup
• Cast Iron or Steel rings

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Force

• Net extension force PcaoAcap PannulusAannulus
• Often we will ignore back pressure
• Knowing when NOT to ignore back pressure is
  important
• Net retraction force PannulusAannulus - PcapAcap

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Cylinder Velocities

• Generally VQ/Area
• V in cm/min
• Q in L/min
• A in cm2 OR
• V in inches/min
• Q in inches3/min
• A in inches2 Note this ignores leakage.

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Special Case

• Regenerative Flow
• Pressure is equal on both ends of cylinder
• Cylinder is effectively reduced to rod size
• Speed is increased
• Force capability is reduced
• Only applies in extension

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Special FeaturesValves

• Pilot Operated Checks
• Counterbalance (More on these later)

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Stop Tubes

• Limit extension
• Can also control buckling in long cylinders

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Stop Valves
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Cushions
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Other Cylinder Issues

• Cavitation
• Can occur in overrunning loads
• Can have same negative effects as in pumps
• May need to protect the load from falling
• We will typically use some type of meter-out
  control to manage this
• Pilot operated check
• Counterbalance
• Simple throttle
• Meter out DCV spool (more on these later)

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Pressure Intensification

• Occurs on the Annular (rod end) side of a double
  acting cylinder when in meter-out control

Load
Pressure?
Flow Control
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Thermal Relief Valves

• Protect against high pressure from thermal
  expansion
• Act as a small pressue relief valve
• Set valve well above expected system pressure

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Special Case of Pressure Intensification Piston
Seal Failure
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Piston Seal Failure in a Meter-Out Application
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Cylinder Sizing Example

• A certain plywood press requires 80 lbs of force
  applied to each square inch of a 4 foot x 8 foot
  sheet of OSB during the gluing cycle
• You are asked to help select the cylinders for
  this application
• The design currently calls for 4 cylinders to
  operate the press
• Pressure for the system is limited by OSHA to a
  maximum of 2400 psi

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Cylinder Sizing Example

• Calculate the total force needed
• 48 wide x 96 long x 80 lbs/in2 368,640 lbs
  force
• Force per cylinder is then 368,640 / 4 92,160
  lbs
• Our system pressure has a limit of 2400 psi
• Fcylinder Pressure x Area or ?P x A
• We will assume ?P is zero unless told otherwise
• Solve the equation for AREA A Fcyl/ ?P
• A 92,160 lbs / 2400 lbs/in2 38.4 in2
• A pD2/4 solve for D Dv(4A/p) 6.99 in

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Whats next?

• Choose a 7 inch diameter cylinder
• If we do operate the press at the full 2400 psi
  what force will the cylinder extert?
• F PA 2400 psi x p(72)/4 92,362 lbs
• Or for the four cylinders on our press.
• F 369,451 lbs
• Press pressure on 4x 8 sheet is 80.17 psi