Paper Example on Forward Reverse Transmission (FRT)

4 pages
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University of California, Santa Barbara
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The purpose of this project is to design a three-wheeled commuter vehicle gearbox. The gear box allows a vehicle top speed in forward (76mph) and the high speed in reverse (38mph). The engine horsepower, torque outputs, and target design life are known. The engine, has a maximum speed of (3,600rpm), a speed increase attached to the output of the engineer (ratio of 1:1.50), the CVT speed ratio range (0.78:1 for lowest speed, and 3.49:1 for highest speed), and the rear wheel/tire size. We designed the gears and shafts and specified the bearings, showing the fundamental equations we used and the resulting design values determined for gears, shafts, and bearings, including the factors of safety against relevant failure modes.

Description. We Designed gear sets, bearings, and shafts for a Forward/Reverse Transmission (FRT) for a 3-wheeled commuter vehicle utilizing 34 horsepower diesel engine. Speed increase, Constant Velocity Transmission (CVT) drive, and front wheel drive are predefined. The FRT met performance and reliability requirements as expressed in the Project Objectives section below.

Project Objectives:

FRT had one ratio for forward and one ratio for reverse.

Operating ratios allowed the vehicle to travel a maximum of 76 mph when at engine maximum rpm. Top speed in reverse was 36 mph. The engine was equipped with a CVT coupled to a speed increase (that comes with the motor assembly).The maximum input rpm to the FRT was 6900 rpm with the engine at full rpm.

FRT input shaft was 1-1/8 diameter with 1/4 square key (to mount CVT driven pulley) or metric equivalent. Shaft extended out of opposite side of the housing to accommodate the installation of some rotation brake.

Output gear was mounted on the specified Differential.

Output shaft exited both sides of the FRT case to accommodate front wheel drive shafts (FWD) that were mounted on the unit. FRT provided FWD driveshafts similar to what is used on current compact cars (e.g. Toyota Yaris, Chevy Aveo, etc.).

The gears in the FRT that are employed in the forward drive mode used helical gear teeth featuring a 200 helix angle and 200 pressure angle teeth. The gears in the FRT that are applied in the reverse drive mode used either helical gear teeth and met durability requirements.

The vehicle weighed 1600 lbs (loaded) with 67/33 weight distribution front/rear. We assumed the coefficient of friction the between the tire and pavement to be 1.0 for acceleration and cruise.

The weight of the gears, shafts, and bearings of the FRT was kept to a minimum; and no more than 75 lbs total for these parts.

System life met or exceed an equivalent 140,000 miles at 90% reliability.

Shifting was accomplished via cable shifter and shift dog system inside the unit.

FRT volume /geometry was suitable to be mounted between the engine and firewall. It allowed for sufficient room to accommodate the passenger foot wells.

All gears of the FRT were fully encased in a housing.

Project Deliverables:

1. We perform the necessary speed and force calculations to determine the rotation speeds and torques required of the FRT input and output systems, as well as for the components that we designed internally to the FRT which include the gears, shafts, and bearings.

2. The mid-shaft design was capable of allowing for the needed shift components.

3. Our design is suitable to fit into a case that can be mounted behind the specified engine and to some form of a chassis frame.

Power, Torque and Speed Ratios Requirements

Maximum engine output speed = 3600rpm.

Maximum input speed = 6900rpm

Engine Power= 34hp.

HP=T in-lbs*n(RPM)63025Output Torque = 34 * 63025/ 3600

= 595.24

Input Torque = 34 * 63025/ 6900

= 310.56

Gear Specification for spur gear

The number of teeth of the gear z = 20

Tip diameter da = 2.67

Modular m m=daz+2m= 0.12

Span measurement k = 7.6604

Pressure angle = 20o

Shaft Layout

The input and output shafts are held in place by two bearings one on each side. The gears are mounted on the shafts and power is transmitted from the input shaft to the output shaft. We used splines to hold the gears to the shaft. We used Serrations splines where the shafts had diameters that were small and involute splines where the gears had a larger diameter and needed to be strong. Some of the retaining rings that we used were constant, push nut and spiral rings. The bearing was held in place by the press fit (Grote, 103).

Force Analysis

Ks Kt.tana


KtTangential gear load

KsRadial gear load

Shaft Material Selection.

The material that we selected for the shaft is the steel alloy 300m. This material is strong enough to withstand stress and fatigue. The alloy is a composition of two compounds. The compounds are vanadium and silicon (Grote, 65).

Bearing Selection.

We included different types of bearings which served different purposes. Some of the bearings included fixed bearings, float bearings, and rolling bearings. The rolling bearings allowed the shafts to expand from the high engine temperatures. We used the horizontal bearings to allow the radial motion and the fixed bearing to allow the axial weight.

Key and Retaining Ring Selection.

The retaining rings that we selected were, self-locking rings, radial rings on the shaft, axial rings for axial installation and constant section rings which were used for internal components.


We designed the engine to have a capacity that was beyond the normal load. Engine size that is more than the load allowed the engine to have to be stronger and have a factor of safety of 7.

Works Cited

"Bearings - Buy Cylindrical, Needle, Tapered ,Spherical Bearings Online@ Industrybuying." Online Shopping Site - Buy Industrial Tools ,Power Tools & Hand Tools Online,

HPC Gears: Manufacturer And Stockist Of Gear Transmission Products,

Jark-Heinrich, Grote. Springer Handbook of Mechanical Engineering. Springer, 2008.

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