Design And Material Selection Of Gearbox Components

Principle function of Gearbox

The gearbox, also called as gear case, is a gear on a marine system responsible for power transmission mechanism from prime mover into useful work.   

It is set of gear which transmitting power from one shaft to another shaft. The gearbox is used to wide range of application in industrial, automobile, marine line and other machinery applications.

The casing of gear box and gear heads are available in different sizes, shape, capacities and speed ratios. The principle function of the gearbox is to convert electric motor power into an output of lower RPM with high torque.

• The gearbox is accurately assemble to control gear box and shaft alignment.
• The gearbox housing contain gear oil for lubrication and provide cooling effect to rotating part of gearbox assembly.
• The metal casing require to protection of gears and prevent from environment direct contact. The casing also necessary in order to prevent from water and dust particle.

Gearbox Specifications

The gearbox is selection according to application and requirement of torque with respect to speed. There are some important parameters needs to consider before selecting gearbox for specific application.  The parameters are describe as following:

1. Gear ratio
2. Gear arrangement.
3. Reduction ratio or reduction output.
4. Shaft alignment.
5. Maximum input speed
6. Maximum input power.
7. Output torque.

 Discuss about the individual component of gearbox and transmission system associate with the gearbox. The important parameter of gearbox are gears which is essential for all kinds of gearbox.

1. Types of gears :
   1. Spur Gear: parallel teeth to the axis and used o sliding mesh.
   2. Helical Gear: incline gear, inclination angle with the helix.
   3. Double helical Gear: Double helical gear with arrangement of helical teeth.
   4. Epicyclic gear train : spur or helical gear rotate with centers which are not stationary

1. Find Gear ratio of single gear train :

The following equation associate with the gear ratio and velocity ratio between the pair of gear wheel and that have inverse relation to the number of teeth to every teeth.

                             N_B/N_A = D_A/D_B= n_A/n_B

                                                N_B = N_A (n_A/n_B)

Where:

N_A= rev per min of gear A,

 n_A = number of teeth on A

N_B = rev per min of gear B,

 Speed, torque and power ratio of Gear-train

the power induce from the gear train express by the shaft is directly proportional to the speed of revolution.

Then

          T_A N_A = T_B N_B

As per given power supply, torque is inversely proportional to the speed of revolution, if the revolution per minute reduce the torque, the effect will increase the same ratio.

T_B/T_A = n_B/n_A

Where:

          T_A = torque transmitted by A

          T_B = torque transmitted by B

Velocity ratio =

The gear box assembly consist various mechanical component such as gears, shaft, gear box casing, bearings, clutch etc.  The material selection of individual component as per the application in gear box assembly. For this case, 30NiCr1 steel is assign to gear. The numerical digit indicate chemical composition of steel as per weight.  The chemical composition of steel material are as following:

Table. 1 30NiCr1 chemical properties

Material	Weight in %
Carbon	0.31
Si	0.25
Mn	0.48
P	0.008
S	0.002
Cr	0.82
Ni	2.64
Mo	0.06
Cu	0.010

For increase strength of gear body, it is require to perform Heat treatment process such as Hardening and Tempering.

For shaft material C45 quality steel selected for gear box design. The gear shaft material needs to perform case hardened steel process in order to perform heat treatment process.

Gearbox Specifications

The roller bearing have been selected for assembly of shaft and bearing. The selection of bearing such a way that lifetime of the bearing should be  days.

The material selection for gear box casing is GG 20 quality grey cast iron.  The material G-42 can be elected for casing of gear box.

The stainless steel material is used for cone clutch design and manufacturing. 

The gear box contain mainly three shafts such as input shaft, output shaft and counter shaft.  The shaft carries gears which transmit power and it always subjected to bending moment due to external applied load.  Therefore multispeed gearbox at the neural position and that significant through “I”. Therefore layout should be execute to the gears when they cannot engage with each other.  in gearbox, forward and backward motion of one lever engagement with two pairs, whereas other motion along with the forward and backward stoke with remaining two pairs.

Average estimation of the distance between supports X and Y can be taken as 2.5 times of the face widths of the pairs of gears.

Average length of shaft = (20 + 21 + 26 + 30) x 25 = 243 mm

The output shaft and intermediate shaft subjected to maximum bending moment,  the gear pair “EF” is in engagement as it is extreme from supports and has more tangential force through CD pair.

Input shaft is a cantilever and loaded at center of pair “AB”

Reaction obtained through pair AB = force along with pressure angle.

Further reaction through pair EF =

Bending Moment on input shaft =

Due to bending moment, Reaction occur at A & B point,

Reaction at point A,

Axial force =

Reaction at  and  of output shaft,

Axial force =

Reaction at X and Y at countershaft,

Bending moment on main shaft =

Bending moment on intermediate shaft = 2.6 x 1000 x 0.105 = 273 Nm

Torque at input shaft = 80 x 1.25 = 100 Nm

Torque develop on intermediate shaft = 190 Nm

Torque develop on main shaft = 375 Nm

In order to find diameter of shaft, use soderberg failure theory :

Endurance strength = 135 Mpa & Factor of safety = 1.8

The mounting gear require axial locking device in order to stop axial movement over shaft. Therefore circlip and shoulder are provided over the shaft.

In case selection of bearing according to shaft diameter, then 25 mm bore diameter bearing need to select for accurate assembly.

Material Selection

For main shaft diameter,

Bearing selection according to main diameter of shaft.

Approximate 6mm added for sliding gear. Other effect added 4 mm. so average shaft diameter = 35 + 6 +4 = 45 mm

So the standard bearing select as per shaft diameter (45 mm)

For hollow gear to supported at non-rotating axle between X and Y point. The actual Bending moment diagram will be lesser than compare to entire shaft length.

Consider trail value of inner diameter  and outer diameter

The above value consider acceptable for shaft and gear assembly

In gearbox, gear play important role for power transmission from high rpm to low torque and vice versa.  For gear design, material selected as 30NiCr1 which subjected to fatigue cyclic stress. The mechanical properties of that gear are as following:

Ultimate tensile stress =  

Yield stress =

Hardness = 440 BHN.

Service factor = 1.25

1^st pair:

Use Lewis equation for gear:

Input rpm = 1700 rpm

Power = 20.8 kW

Consider fatigue strength of material = 450 Mpa

Minimum numbers of teeth = 16

Factor of safety = 2

Module of gear = 4

Input speed = 1700 rom

Input torque = 150 Nm

Diameter of pinion = 16 x 4 = 64 mm

Velocity =

Velocity ratio =

Tangential force = 4.6875 x 1.25

2^nd pair :

Consider module = 3

Helix angle

Engine speed = 3600 rpm

Find the number of teeth on the gear

Centre distance =

Lewis form factor = 0.3

Tangential force

3^rd pair:

The total number of teeth for gear “C” and “D” = 22+44 = 66 (the module as the pair AB as the total numbers of teeth are same)

For input gear:

Lewis form factor = 0.39

4^th pair :

Lewis form factor = 0.377

Design of gear pair with standard:

In case gears have high precision shaved and ground teeth, then

The following condition needs to satisfy for fatigue life cycle.

It is consider that material react in different way under different conditions, if it is use as machine component. That needs to consider following fatigue strength modifying factors: such as Surface factor, Size factor, Reliability factor, Temperature factor, stress concentration factor and miscellaneous factor.

This factors are used to find the fatigue strength of material, which usually used for reduction of design strength.

1. Load correction factors :
2. Dynamic load factor

Table. 1 Material properties

Size factor	0.9
Surface factor	0.7
Reliability factor	0.86
Temperature factor	1
Stress concentration factor	1
Miscellaneous factor	1

Load modifying factor:

The overload correction factor as. Consider face width = 30mm, .

Design of shafts

From AGMA standard table, J = 0.365

Bending stress =

in order to find fatigue strength of material or life cycle of material, it is require to perform fatigue analysis. The following equation in order to find the surface fatigue strength of steel material. This procedure is not economic and it require sufficient time to obtain result. Stress parameter of material as following:

HB is the brinell harness of material and it is sustain up to  cycles completely reverse stress. In case, according to different standard, if two material which equal chemical properties and different strength then lesser value of strength not accepted. According to AGMA standards suggest that the contact fatigue strength can be modified as per endurance limit of material. The corrected fatigue strength of material

Load stress Factor:

The contact stress are focus on the localized area near the surface contact and then spread over the entire cross section area of element. Therefore, the contact stress value are considerable high then the average value of stress. For consider this relation, it is require to introduce Hertz contact stress theory.  The load stress factor depends on two parameters such as elasticity of material and co-efficient of elastic.  

The co-efficient of elastic

The geometry factor as following:

The contact stress obtained,

The fatigue strength of material:

Design safe.

Engineering works:

For easy running of gear box, the tuning fits with considering small clearance with general requirement for the fit accuracy. The preferred fits for gear box are H8/f8 and H9/f8.

The clearance are provided in entire assembly of gearbox. Minimum 1 mm to 4 mm clearance are provided at male and female part assembly. In gear and bearing assembly 0.75 mm clearance provided, similarly for shaft and bearing 0.75mm clearance provided. For gear box casing 4 mm clearance is assign for smooth operation. For oil seal assembly 0.1 mm clearance provided. Assembly of shaft and gear box casing 1 mm clearance are provided for balancing and axial alignment.

The individual part such as gear, shaft, gear box body and Cone clutch needs to manufacture through conventional manufacturing process.  Other mechanical component such as oil seal, bearing need to purchase from market because it is difficult to manufacture and require accuracy for production also. That consume time and need separate investment for individual component.

For gear box body casting manufacturing process suitable for single component manufacture. The shaft and gears are manufacturing through conventional method such as turning, gridding, cutting, hobbling, teeth cutting, broaching, milling, drilling, threading, knuckling etc.  

Cone clutch design parameter:

Thickness of cone assume = 12 mm

Inner diameter

Outer diameter,

So, gear box require 26.208 kW power output require for satisfied design criteria.

Oil seal and efficiency:

The oil seal are rigidly griped against the rotating shaft in order to prevent leakage from gear box. The energy loss due to friction is depends upon various factors such as speed of shaft, shaft diameter, surface finish and surface roughness. For this case shaft rotating minimum 1700 rpm will generate frictional loss around 18 Watts.  The diameter of oil 100mm. it is common for standard design of gearbox manufacturing that incorporate more than one oil seal on a provided shaft which eliminate leakage of lubrication.  If shaft provided more than one oil seal, that exceeding frictional losses over 110 to 120 watts.

Bearing efficiency:

The friction torque of the bearing size 6211 with 1850 radial load applied upon it.

References

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