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Types of Gear Trains. Simple gear train Compound gear train Planetary gear train. Reverted Gear train. Concentric input & output shafts. Reverted Gear train. The driver and driven axes lies on the same line. These are used in speed reducers, clocks and machine tools.

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types of gear trains
Types of Gear Trains
  • Simple gear train
  • Compound gear train
  • Planetary gear train
reverted gear train
Reverted Gear train

Concentric input & output shafts

reverted gear train1
Reverted Gear train

The driver and driven axes lies on the same line. These are used in speed reducers, clocks and machine tools.

If R and N=Pitch circle radius

& number of teeth of the gear

RA + RB = RC + RD

and NA + NB = NC + ND

planetary gear train
Planetary Gear Train…
  • In this train, the blue gear has six times the diameter of the yellow gear
  • The size of the red gear is not important because it is just there to reverse the direction of rotation
  • In this gear system, the yellow gear (the sun) engages all three red gears (the planets) simultaneously
  • All three are attached to a plate (the planet carrier), and they engage the inside of the blue gear (the ring) instead of the outside.
planetary gear train1
Planetary Gear Train…
  • Because there are three red gears instead of one, this gear train is extremely rugged.
  • planetary gear sets is that they can produce different gear ratios depending on which gear you use as the input, which gear you use as the output, and which one you hold still.
planetary gear train2
Planetary Gear Train…
  • They have higher gear ratios.
  • They are popular for automatic transmissions in automobiles.
  • They are also used in bicycles for controlling power of pedaling automatically or manually.
  • They are also used for power train between internal combustion engine and an electric motor
epicyclic gear train
Epicyclic Gear train

Epicyclic means one gear revolving upon and around another. The design involves planet and sun gears as one orbits the other like a planet around the sun. Here is a picture of a typical gear box.

This design can produce large gear ratios in a small space and are used on a wide range of applications from marine gearboxes to electric screw drivers.

epicyclic gear train1
Epicyclic Gear train

A small gear at the center called the sun, several medium sized gears called the planets and a large external gear called the ring gear.

epicyclic gear train2
Epicyclic Gear train

Planetary gear trains have several advantages. They have higher gear ratios. They are popular for automatic transmissions in automobiles. They are also used in bicycles for controlling power of pedaling automatically or manually. They are also used for power train between internal combustion engine and an electric motor.

epicyclic gear train3
Epicyclic Gear train

It is the system of epicyclic gears in which at least one wheel axis itself revolves around another fixed axis.

epicyclic gear train4
Epicyclic Gear train

Basic Theory

The diagram shows a gear B on the end of an arm. Gear B meshes with gear C and revolves around it when the arm is rotated. B is called the planet gear and C the sun.

epicyclic gear train5
Epicyclic Gear train

Basic Theory

Observe point p and you will see that gear B also revolves once on its own axis. Any object orbiting around a center must rotate once. Now consider that B is free to rotate on its shaft and meshes with C.

epicyclic gear train6
Epicyclic Gear train

Basic Theory

Suppose the arm is held stationary and gear C is rotated once. B spins about its own center and the number of revolutions it makes is the ratio:

B will rotate by this number for every complete revolution of C.

epicyclic gear train7
Epicyclic Gear train

Basic Theory

Now consider the sun gear C is restricted to rotate and the arm A is revolved once. Gear B will revolve

because of the orbit. It is this extra rotation that causes confusion. One way to get round this is to imagine that the whole system is revolved once.

epicyclic gear train8
Epicyclic Gear train

Basic Theory

Then identify the gear that is fixed and revolve it back one revolution. Work out the revolutions of the other gears and add them up. The following tabular method makes it easy.

epicyclic gear train9
Epicyclic Gear train

Basic Theory

Suppose gear C is fixed and the arm A makes one revolution. Determine how many revolutions the planet gear B makes.

Step 1: revolve all elements once about the center.

Step 2: identify that C should be fixed and rotate it backwards one revolution keeping the arm fixed as it should only do one revolution in total. Work out the revolutions of B.

Step 3: add them up and we find the total revolutions of C is zero and for the arm is 1.

example 1
Example 1

A simple epicyclic gear has a fixed sun gear with 100 teeth and a planet gear with 50 teeth. If the arm is revolved once, how many times does the planet gear revolve?

gear ratio velocity ratio
Gear Ratio(Velocity Ratio)

Many machines use gears. A very good example is a bicycle which has gears that make it easier to cycle, especially up hills. Bicycles normally have a large gear wheel which has a pedal attached and a selection of gear wheels of different sizes, on the back wheel. When the pedal is revolved the chain pulls round the gear wheels at the back.

gear ratio velocity ratio1
Gear Ratio(Velocity Ratio)

The reason bicycles are easier to cycle up a hill when the gears are changed is due to what is called Gear Ratio (velocity ratio). Gear ratio can be worked out in the form of numbers and examples are shown. Basically, the ratio is determined by the number of teeth on each gear wheel, the chain is ignored and does not enter the equation.

But WHAT does this mean? It means that

the DRIVEN gear makes TWO rotations

for every ONE rotation of the Driving Gear.

gear ratio examples
Gear Ratio - Examples

What does this mean? For every 3

rotations of the driving gear, the driven

gear makes one rotation.

gear ratio examples1
Gear Ratio - Examples

What does this mean? For every 4 rotations

of the driving gear, the driven gear makes 1

rotation.

working out rpms revolutions per minute
Working out RPMs (revolutions per minute)

In the example shown, the DRIVER gear is larger than the DRIVEN gear. The general rule is - large to small gear means 'multiply' the velocity ratio by the rpm of the first gear. Divide 60 teeth by 30 teeth to find the velocity ratio(1:2). Multiply this number (2) by the rpm (120). This gives an answer of 240rpm

working out rpms revolutions per minute1
Working out RPMs (revolutions per minute)

In the example shown, the DRIVER gear is smaller than the DRIVEN gear. The general rule is - small to large gear means 'divide' the velocity ratio(3:1) by the rpm of the first gear. Divide 75 teeth by 25 teeth to find the velocity ratio. divide the 60rpm by the velocity ration (3). The answer is 20rpm.

working out rpms revolutions per minute2
Working out RPMs (revolutions per minute)

If A revolves at 100 revs/min what is B ?

(Remember small gear to large gear decreases revs)

compound gear ratios
Compound Gear Ratios

When faced with three gears the question can be broken down into two parts. First work on Gears A and B. When this has been solved work on gears B and C.

The diagram shows a gear train composed of three gears. Gear A revolves at 60 revs/min in a clockwise direction.

What is the output in revolutions per minute at Gear C?

In what direction does Gear C revolve ?

compound gear ratios1
Compound Gear Ratios

This means that for every THREE revolutions of GEAR A, Gear B travels once.

Since we are going from a SMALLER gear to a LARGER gear we DIVIDE the

Rpms.

Now find the gear ratio for B & C.

This means for every ONE rotation

of gear B, gear C makes SIX rotations.

is there an easier way
Is there an easier way?

You can also multiply the two gear ratios together to get the TOTAL gear

ratio. In the above figure we see that gear C will make TWO rotations for

every one rotation of gear A. And since gear C is smaller than gear A we

multiply.

compound gear ratios2
Compound Gear Ratios

Below is a question regarding 'compound gears'. Gears C and B represent a compound gear as they appear 'fixed' together. When drawn with a compass they have the same centre. Two gears 'fixed' together in this way rotate together and at the same RPM. When answering a question like this split it into two parts. Treat gears A and B as one question ANDC and D as the second part.

What is the output in revs/min at D and what is the direction of rotation if Gear A rotates in a clockwise direction at 30 revs/min?

compound gear ratios3
Compound Gear Ratios

Considering that Gear B is smaller than Gear A we can conclude that the

RPMs for gear B is 30*3 = 90 rev/min

Since Gear B is at 90rev/min and has the SAME rotational speed as gear C

Multiply by 4 to get Gear D’s speed. Thus, Gear D moves at 90*4 = 360 rev/min

Since Gear A moves at 30rpms and Gear D is SMALLER. We multiply by 12. 30*12 = 360 rev/min

OR

answer
Answer

If Gear A turns CCW, then gear B

turns CW along with gear C as they are a compound gear. Therefore, Gear D rotates CCW.

slide44
Gear

The gear ratio of this gear box is 75 to 1

That means the last axle rotates 75 times slower than

the first axle. It also means the last axle has 75 times

the torque as the first axle.

slide45

Try this experiment.

Have one person turn

this wheel.

And have another person

try to hold on to this wheel.

slide46

Gears can increase the torque (and force) that they exert

on something. This is known as mechanical advantage.

torque increases

BUT, it comes at a price. Do you know what it is?