actuators - introduction

ACTUATORS

Introduction

• Actuators extract energy from a fluid and convert it to mechanical energy to perform useful work.

• Linear actuators – Cylinders

• Rotary actuators – Motors

• Limited rotation motors – Oscillation fluid motors

Linear Hydraulic Actuators

• Single acting cylinder– Consists of a piston inside a cylindrical housing called a

“Barrel”.

– Attached to one end of the piston is a rod, which extends outside one end of the cylinder (rod end).

– At the other end (blank end) is a port for the entrance and exit of oil.

– Exerts a force in only one direction.

– Retraction can be accomplished by gravity or by the inclusion of a compression spring in the rod end.


• Single acting cylinder


• Double acting cylinder– Capable of delivering forces in both the directions.

– Barrel: Seamless steel tubing, honed to a fine finish on inside surface

– Piston: Ductile iron, contains U-cup packings to seal leakage between piston and barrel.

– The ports are located in the end caps which are secured to barrel by tie rods.

– The load of the piston rod at the neck is taken by a rod bearing, which is made of brass or bronze.

– Rod wiper is provided at the end of the neck to prevent foreign particles and dust from entering into the cylinder along with the piston rod.


• Double acting cylinder


• Cylinder mountings and mechanical linkages


• Cylinder Force, Velocity and Power– Force (F) and Velocity (v) of a double acting cylinders are not same

for extension and retraction strokes.

• Ap - Area of Piston

• Ar - Area of Rod

– Extension stroke

• Fext = P × Ap

• vext = Qin/Ap

– Retraction stroke

• Fret = P × (Ap – Ar)

• vret = Qin/(Ap – Ar)


• Cylinder cushions– To slow down the piston near the ends of the stroke. This prevents

excessive impact when the piston is stopped by the end caps.

– Deceleration starts when the tapered plunger enters the opening in the cap.

– This restricts the exhaust flow from the barrel to the port.

– During the last small portion of the stroke, the oil must exhaust through an adjustable opening.

– The check valve allow the free flow to the piston during direction reversal.

Linear Hydraulic ActuatorsCylinder cushions

Exercises • A pump supplies oil at 20 gpm to a 2-in diameter double-acting hydraulic

cylinder. If the load is 1000lb.(extending and retracting) and the rod diameter is 1in, find

– The hydraulic pressure during the extending stroke [Ans: 318psi]

– The piston velocity during the extending stroke [Ans: 2.05ft/s]

– The cylinder horsepower during the extending stroke [Ans: 3.72 HP]

– The hydraulic pressure during the retraction stroke [Ans: 425 psi]

– The piston velocity during the retraction stroke [Ans: 2.73 ft/s]

– The cylinder horsepower during the retraction stroke [Ans: 4.96 HP]

Note:

Power (KW) = vp (m/s) × F (KN) = Qin (m3/s) × p (kPa)

Exercises

• Find the cylinder force F required to move a 6000-lb weight ‘W’ along a horizontal surface at a constant velocity. The coefficient of friction (CF) between the weight and horizontal support surface equals 0.14. [Ans:840lb]

• Find the cylinder force F required to lift the 6000-lb weight ‘W’ along a direction which is 30o from the horizontal. The weight is moved at constant velocity. [Ans: 3000lb]

• The 6000-lb weight is to be lifted upward in a vertical direction. Find the cylinder force required to

– Move the weight at constant velocity of 8 ft/s. [Ans: 8980 lb]

– Accelerate the weight from zero velocity to a velocity of 8 ft/s in 0.50s. [Ans: 16ft/s2 , Faccel = 2980 lb]

Cylinder loadings through Mechanical linkages

• First-class lever system


• Second - class lever system


• Third-class lever system

Exercises• Calculate the cylinder force required to overcome the load force for first,

second and third class lever using the following data: L1 = L2 = 10in., Φ = 0o, Fload = 1000 lb . [Ans: 1000 lb, 500 lb, 2000 lb]

• A pump delivers oil at a rate of 18.2gpm into the blank end of the 3-inch diameter hydraulic cylinder shown in fig. The piston contains a 1-inch diameter cushion plunger that is 0.75 inch long, and therefore the piston decelerates over a distance of 0.75 inch at the end of its extension stroke. The cylinder drives a 1500-lb weight, which slides on a flat horizontal surface having a coefficient of friction (CF) equal to 0.12. The pressure relief valve setting equals 750psi. Therefore, the maximum pressure (p1) at the blank end of the cylinder equals 750psi while the cushion is decelerating the piston. Find the maximum pressure (p2) developed by the cushion.[Ans: 856psi]


Double Rod Cylinder•Cylinder with single piston with piston rod extending from each end.•Cylinder allows work to be performed at either or both ends.•It may be desirable when operating speed and return speed are equal.


Tandem Cylinder•Has two cylinders mounted in line with pistons connected by a common piston rod.•These cylinders provide increased output force when the bore size of a cylinder is limited.•But the length of the cylinder is more than a standard cylinder and also requires a larger flow rate to achieve a speed because flow must go to both pistons.


Telescopic Cylinders•These cylinders are where long work-strokes are needed.

Rotary Hydraulic Actuators


• Analysis of Torque Capacity– RR = outer radius of rotor (in, m)

– RV = outer radius of vane (in, m)

– L = width of vane (in, m)

– P = hydraulic pressure (psi, Pa)

– F = hydraulic force acting on vane (lb, N)

– A = surface area of vane in contact with oil (in2 , m2)

– T = torque capacity (in. lb, N .m)


• Analysis of Torque Capacity– F = P A = P (RV - RR ) L

– T = P (RV - RR ) L ((RV + RR ) /2) = (PL/2) (R2V - R2

R )

– VD = π(R2V - R2

R ) L

– T = (P VD /2 π)

Hydraulic Motor theoretical torque, Power and Flow rate

English Units Metric Units

Theoretical Power (W) =

Hydraulic Power = Hydraulic Power =

Motor Efficiencies

• Volumetric efficiency (ηv)

• Mechanical efficiency (ηm)

• Overall efficiency (ηo)

Exercises

• A single-vane rotary actuator has the following physical data: outer radius of rotor = 0.5 inch, outer radius of vane = 1.5 inch, width of vane = 1 inch. If the torque load is 1000 in.lb, what pressure must be developed to overcome the load? [Ans: 1000psi]

• A hydraulic motor has a 5in3 volumetric displacement. If it has a pressure rating of 1000 psi and it receives oil from a 10-gpm theoretical flow rate pump, find the motor (a) speed, (b) Theoretical torque and (c) Theoretical horsepower. [Ans : 462rpm, 795 in.lb, 5.83HP]

• A hydraulic motor has a displacement of 10in3 and operates with a pressure of 1000 psi and a speed of 2000 rpm. If the actual flow rate consumed by the motor is 95 gpm and the actual torque delivered by the motor is 1500in.lb, find : (a) ηv (b) ηm (c) ηo (d) the actual horsepower delivered by the motor. [Ans: 91.1%, 94.2%, 85.8%, 47.6 HP]

Exercises

• A hydrostatic transmission, operating at 1000psi pressure, has the following characteristics:

Find the (a) Displacement of the motor , (b) Motor output torque

[Ans : 4.71 in3 , 674 in.lb ]

Pump Motor

VD = 5in3 VD =?

ηv = 82% ηv = 92%

ηm =88% ηm = 90%

N=500rpm N=400rpm

actuators - introduction

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