Gears

Question 1

Why do we use gears?

Answer 1

As the simplest form of mechanism to transmit rotational speed from one shaft to another. They will allow us to change the angle of the power transmission, transmit torque and to engage other elements of the power transmission system.

Question 2

How do we prevent slip in gears?

Answer 2

By having meshing teeth.

Question 3

What is the pressure line between two gear teeth?

Answer 3

It is the point of contact where force is transmitted from one gear to the other.

Question 4

What is the pressure angle?

Answer 4

It is the angle between the direction of the force applied and the velocity of the gear.

Question 5

What is the pinion and what is the gear?

Answer 5

The pinion is the smaller of the two gears, the one with the smaller diameter. This makes the gear the one that has the larger diameter.

Question 6

What is the equation for determining shaft speed ratios?

Answer 6

Wp / Wg = -Ng / Np = -Dg / Dp
Here, W is the angular velocity, N is the number of teeth per gear and D is the pitch diameter. The negative signs indicate that the gears rotate in the opposite direction to each other.

Question 7

What is the equation for determining the shaft torque ratios?

Answer 7

Tp/Tg = Wg / Wp

Where T is the output torque. It is simply the reciprocal of the angular speed ratio.

Question 8

How do we calculate Fp, the force acting on a Pinion from meshing with a gear?

Answer 8

Fp = Power / V. Power is the power transmitted by the gear and V is the velocity of the gear.

Question 9

How do we calculate the Pitch Line Velocity of a gear?

Answer 9

V = (Dp / 2) x W

Question 10

What is a single degree of freedom gear train?

Answer 10

A gear train that has one input that we control and one output that we can utilise.

Question 11

What does the speed ratio of a single degree of freedom gear train depend on?

Answer 11

It only depends on the size of the first and the last teeth of the gears. This means that none of the gears in between the first and the last have any affect on the overall speed change. This is why single degree of freedom gear trains aren’t that common.

Question 12

What is a compound gear train?

Answer 12

It is one that has at least one shaft that carries more than one gear on it. The intermediate gear ratios do not cancel each other out and so we can achieve larger overall ratios.

Question 13

In a compound gear train, what do we know about gears on the same shaft?

Answer 13

They will have the same angular velocity.

Question 14

What is a spur gear?

Where do we use spur gears?

Answer 14

It is the cheapest type of gear that has straight teeth which allow running engagement and disengagement.
We use spur gears in automotive transmissions, machine and power tool drives and conveyor systems.

Question 15

What is a helical gear??

Answer 15

They have teeth that are angled at around 15-30 degrees. They are stronger and operate more quietly than spur gears. They will produce an axial load onto their shafts.

Question 16

How do we remove the axial load that helical gears apply to the shaft?

Answer 16

With herringbone gears. These gears are just two helical gears of the same pitch and diameter joined back to back. It removes the need for thrust bearings in machinery.

Question 17

What is a rack and pinion?

Answer 17

It is used to convert rotational motion to linear motion. One gear is now flat.

Question 18

What are bevel gears?

Answer 18

These are gears that aren’t parallel to each other. They are commonly used in differential drives.

Question 19

What are spiral bevel gears?

Answer 19

They are the same as bevel gears, except there teeth spiral into the centre to give greater strength and quieter operation.

Question 20

What are crossed axis helical gears?

Answer 20

These are helical gears that do not run through the same axis. Often they are 90 degrees to each other, but can be made to accomodate a range of different shaft angles.

Question 21

What is a cylindrical worm gear and what do we use the mfor?

Answer 21

It is a cylindrical helical gear with one or more teeth that will mesh with a normal gear. It is capable of high speed reduction and high load application. It generates a lot of frictional heat so constant lubrication is required.
We use them for steering, winch blocks and low speed gearboxes.

Question 22

What type of gear do we use in the final drive of front engine rear wheel drives?

Answer 22

Hypoid gears.

Question 23

What is a two degree of freedom gear train?

Answer 23

Something like an epicyclic or planet gear system. This is when one gear is fixed, and another gear rotates about it. Finally we have a third larger ring gear that is fixed about the point of rotation as the first gear. These devices require two inputs to get a single output.

Question 24

Why do cars require gear boxes?

Answer 24

Because IC engines idle at 1500-5000rpm. This will produce low torque, and when they do produce enough torque there rotational speed is too low.

Question 25

What is a synchromesh gear box?

Answer 25

It is a new type of manual transmission. It allows all gear pairings on the various shafts to remain meshed together. Each gear on the main shaft will rotate freely until engaged by a synchromesh device.

Question 26

What is the diametral Pitch of a gear?

Answer 26

It is the ratio of the number of teeth to the pitch diameter in inches! Pd = N/d For mm, we use m = d/N

Question 27

What is a reverted gear train?

Answer 27

It is a compound gear train that is used when there is small space between the input and output shafts are large changes in speed and power are needed.
The input and output shafts may be parallel with each other and the distance between the two centres of each gear pair must be the same.

Question 28

How does a synchromesh work?

Answer 28

The gears on the output shaft are all loosely connected to the shaft, so that they don’t affect the rotation of the shaft unless they are connected to the shaft. The gears will have a synchroniser cone teeth arrangement. A hub is then fixed to the shaft. Use a sleeve that is free to slide over the hub. Have a synchroniser ring in between the gear and the hub. This will slow the gear down enough to allow the sleeve to slide on top of it and efficiently connect the gear to the shaft.

Question 29

Why do we need a synchroniser ring in a synchromesh gear system?

Answer 29

Because when we want to slide the sleeve over the gear and the hub, since the hub is connected to the shaft it will be rotating at different speeds to the gear it wants to connect to. The synchroniser ring is free to slide axially along the shaft. When we press the clutch, all power is disconnected to the gear, and the sleeve will push the ring against the gear. Through friction, these two will attain the same rotational speed and this will make it easier for the sleeve to connect to the gear.

Question 30

What are the advantages of planetary gear trains?

Answer 30

Provide high power density and reduction. Multiple kinematic combinations. Purely torsional reactions, coaxial shifting. Easy to change ratios.

Question 31

What are the disadvantages of planetary gear trains?

Answer 31

High bearing loads
Constant lubrication requirements
Inaccessibility
Design complexity

Question 32

What are the two types of breaking mechanisms that we can use for planetary gear boxes?

Answer 32

Ring gear braking which is when we use a band to slow the ring gear down
Sun gear braking

Question 33

What do we use a differential gear box for?

Answer 33

To slow down the speed of a wheel when we are cornering.

Question 34

How does a differential work?

Answer 34

It uses planetary gears. The power from the engine is transmitted from the drive shaft to the ring gear through a pinion. A spider bevel gear is attached to the ring gear. The spider gear is the in turn connected to two other bevel gears. So when the car is going straight, the spider gear will simply push the two bevel gears at the same speed to give the wheels the same rotation. But when we want to turn the corner, the spider gear will start to spin on its own axis, thus turning one wheel faster than the other.

Question 35

What is a limited slip differential?

Answer 35

It prevents one wheel being allocated excessive amounts of power, so it keeps both wheels in powered rotation. this ensures that traction will not be limited to the wheel that can handle the minimum amount of power.