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Chapter 5 Hydraulic cylinders

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rotary (motor) Actuators single acting linear(cylinder) double acting ‧ single acting 1) small rod 2) rams ‧ double acting ‧ Telescope-cylinder ‧ Limited angle rotary cylinder § Design consideration of hydraulic cylinders 1.Output force F OP =F h - F b - F f ± F g

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Where 2~6 bar 2.Cylinder as a pushing unit 3.Cylinder as a pulling unit

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4.Calculation of piston rod diameter (d) a)with respect to material strength b)with respect to velocity ratio of the piston ‧ Given 〝 D 〞, 〝 〞 Calculate 〝 d 〞 Material strength Factor of safety

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5.Check for buckling rupture Def: slenderness ratio = where : stroke of piston (i.e. length of piston rod) (a) if >10, then buckling rupture of the piston rod must be considered, when the compression force is applied to the piston rod. where F b :buckling load n :Factor of safety F b = f(mounting situation, slenderness ratio, … ) diameter of piston rod area moment of inertia

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(b) 〝 empirical eqns 〞 to determine F b : i) where s : material strength m : const (for steel m 85) a : const (for steel a ) : const (function of mounting situation) ii) where E: Young ’ s module (for steel 2.1 10 6 bar) I : area moment of inertia(for solid rod )

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6.Determination of the barrel thickness From the formula based on thin pipe: where P : max pressure D : piston diameter S : barrel thickness § End-position cushioning devices — preventing impact at the end of the stroke ‧ Types:1. Cushioning device (inside the cylinder) 2. Shock absorber (outside the cylinder) ‧ Consider Type1(Cushioning device inside the cylinder): Assume:pressure P during the deceleration is const. material strength Safety factor

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where S : stroke ( ) : initial velocity : final velocity Force required to decelerate the mass m from is: Ex: Compute the force required to decelerate a mass m=3.63 kg from = m/min to 0 within a distance of m:

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緩衝裝置 說明： (1) 液缸右側緩衝裝置在左側也有 ( 只是 圖中未示出 ) 。 (2) 液缸左側之 check valve 目的在導引 高壓油 ( 當液缸右行伸出 ) 至整個活 塞面積。 (1) Cushioning device Check valve (2) Shock absorber

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5-4 液壓缸之密封圈 (seals) ‧常見之密封單元： (1): 隔離密封圈 ( 主在阻 絕外在之塵污 ) (2): 凹槽型密封圈 ( 不需 O 型 環 ) (3): 支持環 ( 防止徑向負荷造 成之卡死 ) (4): O 型環 +PTFE 密封圈

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‧ O 型環本身一般只當作 靜態密封之用 ‧加工配合之方法一般可 參考廠商之型錄

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