Original Article
Proc IMechE Part K: J Multi-body Dynamics 2016, Vol. 230(4) 379–386 Design of Scotch yoke mechanisms with ! IMechE 2015 Reprints and permissions: improved driving dynamics sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1464419315614431 pik.sagepub.com
Vigen Arakelian1,2, Jean-Paul Le Baron2 and Manuk Mkrtchyan1
Abstract Input torque balancing through addition of an auxiliary mechanism is a well-known way to improve the dynamic behavior of mechanisms. One of the more efficient methods used to solve this problem is creating a cam-spring mechanism. However, the use of a cam mechanism is not always possible or desirable because of the wear effect due to the contact stresses and high friction between the roller and the cam. The Scotch yoke mechanism is most commonly used in control valve actuators in high-pressure oil and gas pipelines, as well as in various internal combustion engines, such as the Bourke engine, SyTech engine and many hot air engines and steam engines. This mechanism does not create lateral forces on the piston. Therefore, the main advantages of applications include reducing friction, vibration and piston wear, as well as smaller engine dimensions. However, the input torque of the Scotch yoke mechanism is variable and can be balanced. This paper proposes to balance the input torque of Scotch yoke mechanisms without any auxiliary linkage just by adding linear springs to the output slider. It is shown that after cancellation of inertial effects the input torque due to friction in joints becomes constant, which facilitates the control of the mechanism. An optimal control is considered to improve the operation of balanced Scotch yoke mechanisms. The efficiency of the suggested technique is illustrated via simulations carried out by using ADAMS software.
Keywords Scotch yoke mechanism, input torque, balancing, optimal control, dynamics
Date received: 16 May 2015; accepted: 6 October 2015
Introduction applications include reducing friction, vibration and It is obvious that whatever the power of control, even piston wear, as well as smaller engine dimensions. today, one cannot correctly operate a machine with The analysis of a Scotch yoke mechanism shows poor mechanics. If the input torque, that is, the torque that its input torque is highly variable. The input ensuring the constant speed, is highly variable, the torque may be reduced by optimal redistribution of resulting drive speed fluctuation will be substantial. moving masses.7–11 or by using non-circular gears.12 Therefore, highly variable input torques might excite One of the more efficient methods used to solve the torsional vibration, while input torques with frequent problem of input torque balancing is creating a cam- sign changes present a very unfavorable loading case spring mechanism, in which the spring is used to for the gears that are possibly present between the absorb the energy from the system when the torque mechanism and its driving actuator. is low, and release energy to the system when the This paper provides a simple and efficient input required torque is high. It allows reducing the fluctu- torque balancing method, which can be applied to ation of the periodic torque in the high-speed mech- Scotch yoke mechanisms. The Scotch yoke mechan- anical systems.13–21 ism is subject to a wide range of applications and The input torque balancing technique proposed in various publications have been devoted to its this paper is achieved by adding linear springs. study.1–5 This mechanism is most commonly used in control valve actuators in high-pressure oil and gas pipelines, as well as in various internal combustion engines, such as the Bourke engine, SyTech engine 1IRCCyN, Nantes Cedex, France and many hot air engines and steam engines. It is 2I.N.S.A. Rennes, Rennes, France also used in testing machines to simulate vibrations 6 Corresponding author: having simple harmonic motion. The Scotch yoke Vigen Arakelian, I.N.S.A., Rennes 20, avenue des Buttes de Coesmes CS mechanism does not create lateral forces on the 70839, Rennes, F-35708, France. piston. Therefore, the main advantages of Email: [email protected]
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Figure 2. Input torque of a Scotch yoke mechanism.
where IS1 is the axial inertia moment of link 1; mi are Figure 1. Scotch yoke mechanism. the masses of the corresponding links (i ¼ 1, 2, 3); rS1 is the distance between the centre of the joint O and Input torque of a Scotch yoke the centre of mass S1 of link 1. mechanism Substituting equation (5) into equation (1), the input torque of the mechanism is The Scotch yoke mechanism is a reciprocating motion 2 2 mechanism, converting the linear motion of a slider MIN ¼ 0:5m3lOAðÞ’_ sin 2’ ð6Þ into rotational motion of a crank or vice versa (Figure 1). In the present study, it has been considered The obtained result shows that the input that the gravitational forces are perpendicular to the torque of a Scotch yoke mechanism varies according motion plane. to sin2u (Figure 2). As is mentioned by Berkof,7 the input torque of a It means that the average value of the input torque single degree of freedom mechanism due to inertial is equal to zero, and the correction moment created by effects can be found from equation the spring system should be similar to the input torque of the mechanism. Thus, in balancing the system for 1 dT the periods ’ 2 ½ 0; =2 and ’ 2 ½ ; 3 =2 the spring M ¼ ð1Þ , IN ’_ dt must to absorb and accumulate the energy from the Scotch yoke mechanism because the input torque is where T is the total kinetic energy of the mechanism low. With regard to the periods ’ 2 ½ =2; and and ’_ is the input angular velocity. ’ 2 ½ 3 =2; 2 , the spring should release energy to The relationship between the rotation of link 1 and the Scotch yoke mechanism because the required the translation of link 3 can be written as torque is high. It should be noted once again that in the present s ¼ lOA sin ’ ð2Þ paper, the input torque due to inertial effects is con- sidered. In the case of the presence of combustion where ’ is the rotating angle of link 1; lOA is the length forces, the input torque balancing will be different. of link 1, i.e. the distance between the joints O and A; For the case of engines please see combustion-induced s is the translational displacement of slier 3. torque variation in literature.22 The slider velocity can be found by differentiating equation (2) Input torque balancing s_ ¼ lOA’_ cos ’ ð3Þ The spring system should ensure the following Considering that the input angular velocity is con- condition stant and differentiating equation (3), the slider accel- eration can be written as Fspdx þ MINd’ ¼ 0 ð7Þ 2 s€ ¼ lOAðÞ’_ sin ’ ð4Þ
The kinetic energy of the mechanism can be where Fsp ¼ kx is the elastic force of the spring; k is written as the stiffness coefficient of the spring, x is the displace- ment of the spring. 2 2 2 2 2 T ¼ 0:5ðÞ’_ IS1 þ m1rS1 þ m2lOA þ m3lOA cos ’ It should be noted that Fsp has a minus sign during ð5Þ the accumulation of energy and a plus sign during the restitution of energy.
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For the period of the accumulation of potential energy Z ’ Aaccum ¼ MINd’ Z 0 ’ 2 2 ¼ 0:5m3lOAðÞ’_ sin 2’d’ 0 ¼ 0:5m l2 ðÞ’_ 2sin2 ’ ð8Þ 3 OA Figure 3. Displacements of the balancing spring.
The maximal value of the accumulate potential With regard to the spring it can be written energy for both periods mentioned above is Z xmax Z 2 2 =2 Arest ¼ kxdx ¼ 0:5kðxmax x Þð17Þ x Amax ¼ MINd’ Z 0 =2 or 2 2 2 2 ¼ 0:5m3lOAðÞ’_ sin 2’d’ ¼ 0:5m3lOAðÞ’_ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 2Arest ð9Þ x ¼ x2 ð18Þ max k
Integrating equation (7) for the period of energy To determine the displacement x of the spring for accumulation, the following relationship can be the period of energy restitution, let us introduce equa- obtained tion (14) and equation (16) into equation (18) Z Z ’ x ¼ ð Þ Aaccum ¼ MINd’ ¼ kx dx ð10Þ x xmax sin ’ 19 0 0 and So for two periods, accumulation and restitution, displacements of the spring are same, see Figure 3. 2 Aaccum ¼ 0:5kx ð11Þ The proposed traditional solution for input torque balancing in Scotch yoke mechanism is to add a From which cam to input crank in order to execute harmonic dis- rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi placements of the spring. However, taking into 2 account particularities of the Scotch yoke mechanism, x ¼ A ð12Þ k accum it will be shown that a simple balancing technique of the input torque can be found. when x ¼ xmax, the accumulation of energy becomes Let us now turn our attention to the displacements maximum of slider 3. The displacements of the slider vary with the sinusoidal law. Therefore, it is possible to balance 2 Amax ¼ 0:5kxmax ð13Þ the input torque of a Scotch yoke mechanism by adding linear springs between the frame and output Now, from equation (9) and equation (13) the stiff- slider 3. The added springs should ensure the condi- ness coefficient of the spring can be determined tion: xmax ¼ lOA. Thus, by adding simple linear springs the input m l2 ðÞ’_ 2 torque due to the inertial forces will be fully cancelled. k ¼ 3 OA ð14Þ 2 Although the described solution is very simple, this is xmax the first time it is proposed. To determine the displacement x of the spring let us introduce equation (8) and equation (14) into equa- The input torque due to friction in joints tion (12) Let us now consider a Scotch yoke mechanism taking x ¼ xmax sin ’ ð15Þ into account the friction in the mechanism’s joints. Let us now consider the period of energy restitution. Several friction models have been proposed having In this case, the following expression concerning different levels of accuracy, and wide variety of con- input torque can be written as trol solutions have been developed for its 23–30 Z compensation. ’ 32 2 2sin 2’ 2 2 2 In Sawyer et al., a nearly ideal two-dimensional Arest ¼ m3lOAðÞ’_ d’ ¼ 0:5lOAðÞ’_ cos ’ =2 2 Scotch yoke mechanism was constructed to test a ð16Þ model of wear depth as a function cycle number. The model originally developed by Blanchet33 was
Downloaded from pik.sagepub.com by guest on December 5, 2016 382 Proc IMechE Part K: J Multi-body Dynamics 230(4) non dimensionalized and simplified under conditions The input torque due to friction in joints of large numbers of cycles. Experiments, given in lit- erature,32 showed a linear progression of wear over The differential equation describing the motion of the two distinct regions, suggesting a sudden transition Scotch yoke mechanism without linear springs to the in wear modes just after 1.5 million cycles. output slider is given by The review showed that friction must be considered Âà 2 2 2 2 in dynamic models in order to optimally control ðtÞ¼ IS1 þ m1rS1 þ m2lOA þ m3lOA cos ’ðtÞ ’€ðtÞ mechanisms. 1 2 2 In the present paper, the friction model developed m3l ’_ ðtÞ sin 2’ðtÞð22Þ 2 OA by Wilson and Sadler34 has been used. The choice of this model is due to the fact that it provides analytical The joint variable is ’ðtÞ and the control torque results. It allows authors to keep the principal struc- is ðtÞ ture of the paper with only analytically tractable The parameters of the Scotch yoke mechanism are solutions. After torque balancing described above, the reac- m l2 l I ¼ 1 OA ; r ¼ OA ; l ¼ 0:1m; tion forces in prismatic joints are cancelled, i.e. S1 12 S1 2 OA ð23Þ ¼ ¼ F23 F03 0, where F23 is the reaction force between m1 ¼ 3kg; m2 ¼ 0:5kg; m3 ¼ 5kg links 2 and 3; F03 is the reaction force between link 3 and the frame (denoted as ‘‘0’’). In order to simplify the expression of ðtÞ, please With regard to the reactions in revolute joints, they note that int are constant due to the condition: F21 þ F01 þ F1 ¼ 0, 8 Âà 2 2 2 2 where F21 is the reaction force between links 2 and 1; <