Gear Pump Modification.docx

Modification of tooth geometry of gear Pump Today external gear pumps are available with different types of gear such as spur, helical & herringbone gears. Helical and herringbone gears typically offer a smoother flow compared to the spur gears, although all gear types are relatively smooth. Straight spur gears are easiest to cut and are the most widely used. Helical and herringbone gear run more quietly but cost more. They are typically used in large capacity gear pumps. Displacement volume of gear pump is directly affected by gear tooth profile. Since the involute gear tooth profile is easily manufactured and the technology for the power transmission gear can be applied, this profile is usually adopted for a low cost gear pump. Generally gears having involute tooth profile are most commonly used for gear pump due to its ease in manufacturing .In an involute gear, the profiles of the teeth are involutes circle. Various Types of Tooth Profiles used

Fig 1. 20° Full Depth Involute Tooth Profile

Fig 2. Continuous contact type gear pump

Tooth Geometry

Fig. 3 Tooth geometry of gear experimental gear pump

Modern fluid control system requires design of gear pumps with predicable features and operational characteristics. Hence, it is of utmost importance to analyse the design parameters influencing the pump characteristics to understand their effect so as to aid the process of improved design of such pumps to be incorporated in industrial applications for dependable and satisfactory performance. After studying on tooth geometry of gears, it has been found that profile of gear tooth affects directly on the performance of gear pump. Hence to improve the performance of gear pump it is necessary to optimize the tooth profile of gear. The pressure angle is the acute angle between the line of action and normal to the line connecting the gear centres. Theoretically, gear manufacturers can produce any pressure angle. However, the most common gears have a 20 degree pressure angle, with 14.5 degree and 25 degree pressure angle as other common options. Increasing the pressure angles increases the width of the base of the gear tooth, leading to greater strength and load carrying capacity. Decreasing the pressure angle provides lower backlash and smoother operation and less sensitivity to manufacturing errors.

Parameters Correction Factor(ζ0 ): Correction factor is the radial displacement of tooth generating profile. Positive correction factor leads to increase the tooth thickness at pitch circle and to decreases at the tip, so that the tooth shape change according to the value of correction factor used.

Fig.4 Different types of tooth profile for different correction factor Pressure angle (γ) Pressure angle is the angle between the common normal to the contacting teeth ( line of action) and the common tangent to the pitch circle of meshing gears.

Fig. 4 Pressure angle of spur gear. Contact ratio (ε) This is one of the important design aspects of spur gear. This is the number which indicates the average number of teeth in contact.

Ideal Delivery of a Gear Pump Ichikawa T. Yamaguchi and Willekens in 1971 developed analytical expression for ideal flow rate and flow pulsation of external gear pump. Qt= 2𝜋b𝑚2 n[Z + (1-

𝜋 2 𝑐𝑜𝑠2 γ

)]* 10−6

12

….(1)

Where, Qt: Theoretical flow rate

Z: No. Of teeth

b: Tooth width

γ= Pressure angle

m: Module n: RPM of pump For unstandardized gear pump having involute profile external spur gear Qt= n𝜋b [( mZ + m ζ + m − m ζ0 −

mζ 2 ) 2

[ 𝑚 (𝑍 + 2ζ0 )/ 2]2 -

𝜋 2 𝑚2 𝑐𝑜𝑠2 γ 6

] * 10−6 …(2)

Where, ζ : Final value of correction factor From above expressions, it can be seen that, Flow rate Qt is proportional with the number of teeth. 𝑄𝑡 ∝ 𝑚2 , where m is module 𝑄𝑡 ∝Pressure angle Pulsation Flow rate factor It is the factor which gives information about fluctuation in the flow rate. Greater the Pulsation Flow rate Factor more is the fluctuation in the delivery of gear pump. Practically lower Pulsation Flow rate Factor is desirable. For Standard Gear F=

𝜋 2 𝑐𝑜𝑠2 γ 4[Z + (1−

……(3)

𝜋2 𝑐𝑜𝑠2 γ )] 12

For Unstandardized gear F=

𝜋 2 𝑐𝑜𝑠2 γ 4( mZ + m ζ + m − m ζ0 −

2

mζ ) 2

𝜋 2 𝑐𝑜𝑠2 γ [ 𝑚 (𝑍+2ζ0 )/ 2]2 −

𝜋2 𝑚2 𝑐𝑜𝑠2 γ ] 12

……………(4)

Conclusion From above expressions, it is observed that flow rate of gear pump can be increased by increasing the pressure angle of gear, no of teeth, width and by increasing positive correction factor. Also contact ratio decreases with increase in pressure angle and tooth correction factor.But we also have some limitation to increase the value of these parameters. Limitations of Increasing Pressure Angle 1) As the pressure angle increases, the separating force increases which is undesirable. 2) As the pressure angle increases the wear rate of gears increases. 3) As the pressure angle increases meshing noise increases. Hence to in order to improve the performance of the gear pump we need to optimize these parameters. So in this project we are going to vary pressure angle of gear and correction factor of gear tooth by keeping other parameters of gear constant.

Procedure 1) Firstly, we find analytical solution for pressure angle and correction factor of gear tooth to get maximum flow rate. For this we will obtain analytical results between flow rate vs pressure angle by keeping other parameters constant. Similarly we very correction factor of gear tooth and find its effect on flow rate of gear pump. 2) After considering disadvantages of changing above parameters we find optimized solution for the above parameters. 3) After getting optimized solution we manufacture gear of above specification. 4) We will test the modified gears practically in our gear pump by replacing standard gears. We will obtain results for different parameters such as output pressure, flow rate, efficiency, pulsation, noise, wear rate of gear pump practically. 5) After obtaining the experimental results for modified gear pump we will compare the result of standard gear pump with modified gear pump.