Project Objectives: Understand rack and pinion gear mechanism. Come up with an innovative solution for building that design as a standard one. Determine the various forces that are meshed within gears. In context of all the gear parameters referring to right selection of material for rack and as well as for pinion. www.postointernational.tk
Need Need is to develop a system for closing and opening of railway crossing gate with rack and pinion mechanism. In order to design that we will have to assume some data as shown:
Gate is to slide on a rail by rail and wheel mechanism.
Sliding path is frictionless.
Two rack and pinion gears are used to slide gate on rail as load will uniformly be distributed mounted on its ends.
Rack and Pinion Gear Design Rack & Pinion Gear A rack and pinion gears system is composed of two gears. The normal round gear is the pinion gear and the straight or flat gear is the rack. The rack has teeth cut into it and they mesh with the teeth of the pinion gear. . Pinion Rack
Basic Mechanism www.postointernational.tk
Rack and Pinion Gear Design Rack & Pinion Gear contd. Rack and pinion gears provide a greater feedback and steering sensation. A well designed mechanism such as the rack and pinion gears save effort and time. .
Rack and Pinion Gear Design Rack & Pinion Gear contd. The rack and pinion is used to convert between rotary and linear motion. Rack and pinion can convert from rotary to linear of from linear to rotary. The diameter of the gear determines the speed that the rack moves as the pinion turns. Rack and pinions are commonly used in the steering system of cars to convert the rotary motion of the steering wheel to the side to side motion in the wheels. .
Rack and Pinion Gear Design Designing Rack & Pinion Gear. Identify a Need Conceptual Design Preliminary Design Final Design Manufacture Sales & Delivery Engineering Design . Designing Rack & Pinion Gear.
Rack & Pinion Rack & PinionTwo types Rack and pinion gears are available in two variations Straight teethorSpur Type Helical teethor Helical Type www.postointernational.tk
Preliminary Design For spur gear design, pinion and rack both have straight teeth or spur geometry of teeth. So for we will first consider the design of pinion as spur gear. www.postointernational.tk
Spur Gear Design and Selection Objectives • Calculate forces on teeth of spur gears, including impact forces associated with velocity and clearances. • Determine allowable force on gear teeth, including the factors necessary due to angle of involute of tooth shape and materials selected for gears. • Understand how lubrication can cushion the impact on gearing systems and cool them. www.postointernational.tk
Specifications for standard Gear Teeth www.postointernational.tk
Forces on Spur Gear Teeth Ft = Transmitted force Fn = Normal force. Fr = Resultant force θ = pressure angle Fn = Ft tan θ Fr = Ft/Cos θ www.postointernational.tk
Forces on Spur Gear Teeth Power, P; = Torque, Combining the above we can write: www.postointernational.tk
Surface Speed Surface speed (Vm) is often referred to as pitch-line speed: www.postointernational.tk
Strength of Gear Teeth Lewis form factor method Lewis equation Fs = Allowable dynamic bending force (lb) Sn = Allowable stress. Use endurance limit and account for the fillet as the stress concentration factor b = Face width (in.) Y = Lewis form factor (From Table) Pd = Diametral pitch www.postointernational.tk
Lewis form factors (Y) Table www.postointernational.tk
Classes of Gears • Transmitted load depends on the accuracy of the gears • Gear Manufacture – Casting – Machining • Forming • Hobbing • Shaping and Planing – Forming www.postointernational.tk
Force Transmitted Transmitted load depends on the accuracy of the gears. A dynamic load factor is added to take care of this. • Ft = Transmitted force • Fd = Dynamic force • Commercial www.postointernational.tk
Classes of Gears Carefully cut Precision Hobbed or shaved www.postointernational.tk
Expected error in tooth profiles www.postointernational.tk
Design Methods • Strength of gear tooth should be greater than the dynamic force; Fs ≥ Fd • You should also include the factor of safety, Nsf. www.postointernational.tk
Service Factors www.postointernational.tk
Face width of Gears Relation between the width of gears and the diametral pitch. www.postointernational.tk
Dynamic Beam Strength of the Gear To increase the dynamic beam strength of the gear – Increase tooth size by decreasing the diametral pitch – Increase face width upto the pitch diameter of the pinion – Select material of greater endurance limit – Machine tooth profiles more precisely – Use proper lubricant and reduce contamination www.postointernational.tk
Buckingham Method of Gear Design • It offers greater flexibility. • Expected error is based on different-pitch teeth. • More conservative design. www.postointernational.tk
Wear strength (Buckingham) Fw = tooth wear strength. Dp = diametral pitch of pinion. Dg = diametral pitch of gear. b = face width. Kg = load stress factor. www.postointernational.tk
Reference www.gearmanufacture.com www.rackandpinion.design.net www.gearfactory.com www.mechanicaldesign.com Wikipedia Machine Design R.S. Khurmi Hand book of mechanical engineering. www.postointernational.tk
Conclusion So far we have been able to calculate the design of pinion only in the next phase of project we will be designing the rack. Passed on fundamental knowledge of rack and pinion gear mechanism and it design procedure & finally selecting the right material for it. www.postointernational.tk
MECHANICAL ENGINEERING DEPARTMENT JazakALLAH-o-Khaira! www.postomotors.tk