Question

Design requirements: You are required to design a gear box to reduce 2,000 rpm to 90-100 rpm while transmitting a power of 18 hp. The gearbox size should be less than 15 in x 15 in base, and 20 in height and input and output shaft must be in-line. The shaft must have an infinite life, while gear and bearing life should be greater than 10,000 hours. The bearings must have a reliability of 96%. You need to perform the following items: 1. Define problem statement (e.g. specify safety factor, expected life, cost, and operating conditions) 2. Identify loading and possible failure for each element 3. Design gears 4. Design input shaft, output shaft and intermediate shaft 5. Design key for gears 6. Design bearings 7. Produce a final drawing with dimensions by SolidWorks

ignore part 1 and 7

Answer 1

1). For designing of gears we have to keep in mind various factors that can effect efficiency of the whole vehicle. Following are some data summary of the necessary points.

A). Basic size and selection

	Spur gear	Helical gear	Straight bevel	Spiral bevel
Max. pitch line velocity	7	10	5	10
Efficiency per mesh	97%	99%	97%	98%
Power to weight ratio	Medium	Medium to high	Medium	Medium to high

B). Torque selection

Example of prime motor	Uniform	Moderate shock	Heavy shock
Electric motor Hydraulic motor Turbine	1	1025	1.75
Multi-cylinder petrol Engine	1.5	1.75	2.25
Single-cylinder petrol engine	1.75	2	2.5

C). Factor of safety

As we know safety factor is defined as the ratio of ultimate stress to working stress. So we have to keep in mind the load, power, r.p.m of etc. of the gear so that safety factor should be as much as we need.

D). Life of gear.

L=(C/P)^p

Lh= 10^6/60n * (C/P)^p hrs

Where,

C= Dynamic basic load rating

P= Equivalent bearing load

n= Rotational speed

D). Cost calculation foe designing of gear box

Shaft - 10 EUR/Kg

Pinion shaft - 16 EUR/Kg

Gear - 20 EUR/Kg

So we can make an estimate hoe much the whole gear box will cost according to the given data

3). Gear designing

A). Module=CP/pi   

CP= Circular pitch

Pi= 3.14

B). Center distance= (D1+D2)/2

C). Gear ratio = m=Ng/Np

D). No. of teeth,N= P*D

E). Whole depth = a+b

F). Addendum = 1/p

G). Dedendum = 1.25/p

H). Clearance = 0.25/p