actuators - introduction.ppt

7/23/2019 Actuators - Introduction.ppt

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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 – scillation fluidmotors



Linear !ydraulic Actuators

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

#$arrel%.

– Attached to one end of the piston is a rod& which extendsoutside one end of the cylinder 'rod end(.

– At the other end ' )lank end( is a port for the entrance and

exit of oil.

– *xerts a force in only one direction.

– Retraction can )e accomplished )y gravity or )y the

inclusion of a compression spring in the rod end.




• "ingle acting cylinder




• +ou)le acting cylinder – Capa)le of delivering forces in )oth the directions.

– $arrel, "eamless steel tu)ing& honed to a fine finish on inside surface

– -iston, +uctile iron& contains /cup packings to seal leakage )etween

piston and )arrel.

– 0he ports are located in the end caps which are secured to )arrel )y tie

rods.

– 0he load of the piston rod at the neck is taken )y a rod )earing& which

is made of )rass or )ron1e. – 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.




• +ou)le acting cylinder




• Cylinder mountings and mechanical linkages




• Cylinder 2orce& 3elocity and -ower

– 2orce '2( and 3elocity 'v( of a dou)le acting cylinders are not same for

extension and retraction strokes.

• A p / Area of -iston

• Ar / Area of Rod

– *xtension stroke

• 2ext 4 - 5 A p

• vext 4 6in7A p

– Retraction stroke• 2ret 4 - 5 'A p – Ar (

• vret 4 6in7'A p – Ar (




• Cylinder cushions – 0o slow down the piston near the ends of the stroke. 0his prevents

excessive impact when the piston is stopped )y the end caps.

– +eceleration starts when the tapered plunger enters the opening in the

cap.

– 0his restricts the exhaust flow from the )arrel to the port.

– +uring the last small portion of the stroke& the oil must exhaust through

an ad8usta)le opening.

–

0he check valve allow the free flow to the piston during directionreversal.




Cylinder cushions



*xercises• A pump supplies oil at 9: gpm to a 9/in diameter dou)le/acting hydraulic

cylinder. If the load is ;:::l).'extending and retracting( and the roddiameter is ;in& find

– 0he hydraulic pressure during the extending stroke <Ans, =;>psi?

– 0he piston velocity during the extending stroke <Ans, 9.:@ft7s?

– 0he cylinder horsepower during the extending stroke <Ans, =.9 !-?

– 0he hydraulic pressure during the retraction stroke <Ans, B9@ psi?

– 0he piston velocity during the retraction stroke <Ans, 9.= ft7s?

– 0he cylinder horsepower during the retraction stroke <Ans, B.D !-?

Eote,

-ower 'FG( 4 v p 'm7s( 5 2 'FE( 4 6 in 'm=7s( 5 p 'k-a(



*xercises

• 2ind the cylinder force 2 reHuired to move a D:::/l) weight GJ along a

hori1ontal surface at a constant velocity. 0he coefficient of friction 'C2(

)etween the weight and hori1ontal support surface eHuals :.;B.

<Ans,>B:l)?

• 2ind the cylinder force 2 reHuired to lift the D:::/l) weight GJ along a

direction which is =:o from the hori1ontal. 0he weight is moved at constant

velocity. <Ans, =:::l)?

• 0he D:::/l) weight is to )e lifted upward in a vertical direction. 2ind the

cylinder force reHuired to

– Move the weight at constant velocity of > ft7s. <Ans, >>: l)?

–

Accelerate the weight from 1ero velocity to a velocity of > ft7s in :.@:s.<Ans, ;Dft7s9 & 2accel 4 9>: l)?



Cylinder loadings through Mechanical linkages

• 2irst/class lever system




• "econd / class lever system




• 0hird/class lever system



*xercises

• Calculate the cylinder force reHuired to overcome the load force for first&

second and third class lever using the following data, L; 4 L9 4 ;:in.&

K 4 :o& 2load 4 ;::: l) . <Ans, ;::: l)& @:: l)& 9::: l)?

• A pump delivers oil at a rate of ;>.9gpm into the )lank end of the =/inch

diameter hydraulic cylinder shown in fig. 0he piston contains a ;/inch

diameter cushion plunger that is :.@ inch long& and therefore the piston

decelerates over a distance of :.@ inch at the end of its extension stroke.

0he cylinder drives a ;@::/l) weight& which slides on a flat hori1ontal

surface having a coefficient of friction 'C2( eHual to :.;9. 0he pressure

relief valve setting eHuals @:psi. 0herefore& the maximum pressure 'p ;( at

the )lank end of the cylinder eHuals @:psi while the cushion is

decelerating the piston. 2ind the maximum pressure 'p9( developed )y the

cushion.<Ans, >@Dpsi?




+ou)le Rod Cylinder •Cylinder with single piston with piston rod extending from each end.

•Cylinder allows work to )e performed at either or )oth ends.

•It may )e desira)le when operating speed and return speed are eHual.




0andem Cylinder •!as two cylinders mounted in line with pistons connected )y a common

piston rod.

•0hese cylinders provide increased output force when the )ore si1e of a

cylinder is limited.

•$ut the length of the cylinder is more than a standard cylinder and also

reHuires a larger flow rate to achieve a speed )ecause flow must go to

)oth pistons.




0elescopic Cylinders•0hese cylinders are where long work/strokes are needed.



Rotary !ydraulic Actuators




Limited angle rotary actuator :




• Analysis of 0orHue Capacity

– R R 4 outer radius of rotor 'in& m(

– R 3 4 outer radius of vane 'in& m(

– L 4 width of vane 'in& m( – - 4 hydraulic pressure 'psi& -a(

– 2 4 hydraulic force acting on vane 'l)& E(

– A 4 surface area of vane in contact with oil 'in9 & m9(

– 0 4 torHue capacity 'in. l)& E .m(




• Analysis of 0orHue Capacity

– 2 4 - A 4 - 'R 3 / R R ( L

– 0 4 - 'R 3 / R R ( L ''R 3 R R ( 79( 4 '-L79( 'R 93 / R 9R (

– 3+ 4 'R 93 / R 9R ( L

– 0 4 '- 3+ 79 (




Rack/and/pinion piston/type rotary actuator,




!elix actuator,




-arallel piston rotary actuator,



!ydraulic Motor theoretical torHue& -ower and

2low rate

English Units Metric Units

0heoretical -ower 'G( 4

!ydraulic -ower 4 !ydraulic -ower 4



Motor *fficiencies

• 3olumetric efficiency 'Nv(

•

Mechanical efficiency 'Nm

(

• verall efficiency 'No(



*xercises

• A single/vane rotary actuator has the following physical data, outer radius

of rotor 4 :.@ inch& outer radius of vane 4 ;.@ inch& width of vane 4 ; inch.

If the torHue load is ;::: in.l)& what pressure must )e developed to

overcome the loadO <Ans, ;:::psi?

• A hydraulic motor has a @in= volumetric displacement. If it has a pressure

rating of ;::: psi and it receives oil from a ;:/gpm theoretical flow rate

pump& find the motor 'a( speed& ')( 0heoretical torHue and 'c( 0heoretical

horsepower. <Ans , BD9rpm& @ in.l)& @.>=!-?

• A hydraulic motor has a displacement of ;:in= and operates with a pressure

of ;::: psi and a speed of 9::: rpm. If the actual flow rate consumed )y

the motor is @ gpm and the actual torHue delivered )y the motor is

;@::in.l)& find , 'a( Nv ')( Nm 'c( No 'd( the actual horsepower delivered )y

the motor. <Ans, ;.;P& B.9P& >@.>P& B.D !-?



*xercises

• A hydrostatic transmission& operating at ;:::psi pressure& has the

following characteristics,

2ind the 'a( +isplacement of the motor & ')( Motor output torHue

<Ans , B.; in= & DB in.l) ?

Pump Motor

3+ 4 @in= 3+ 4O

Nv 4 >9P Nv 4 9P

Nm 4>>P Nm 4 :P

E4@::rpm E4B::rpm

actuators - introduction.ppt

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