Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change...

43
Gear Drives – Chapter 14 • Gears Function of gears – Transmit mechanical power Change rotational speed Change torque Maintain constant speed ratio Where are gears used?

Transcript of Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change...

Page 1: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Gear Drives – Chapter 14

• Gears• Function of gears

– Transmit mechanical power

– Change rotational speed– Change torque– Maintain constant speed

ratio

• Where are gears used?

Page 2: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Topics• Introduction to Gear Drives• Gear Construction• Gear Terminology• Gear Ratio• Types of Gears• Open Gear Alignment• Gear Wear and Failure Analysis• Enclosed Gear Drives• Enclosed Gearbox Types• Gearbox Installation and Troubleshooting

• Credit: Some images in this presentation are in the public domain and were taken from Wikimedia Commons, Wikimedia Foundation.

Page 3: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

• What is a gear?• A toothed part that meshes with another

toothed part to transmit power, change speed, or direction.

- The larger of two interacting gears is referred to as the “Gear”.

- The smaller one is called the “pinion”

Gear Drives MECH1200

Page 4: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

12 Dimensions of a Gear

1.Facewidth2.Circular Tooth Thickness3.Tooth Space Width4.Pressure Angle5.Outer Diamter6.Base Circle Diameter7.Addendum8.Dedendum9.Whole Dpth10.Number of Teeth11.Pitch Diameter12.Diametral Pitch

Page 5: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Circular Pitch – The distance between one point on a tooth and the corresponding point on the next tooth when measured along the pitch circle

Circular Pitch=𝐶𝑖𝑟𝑐𝑢𝑚𝑓𝑒𝑟𝑒𝑛𝑐𝑒𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 h𝑡𝑒𝑒𝑡

=𝜋

𝐷𝑖𝑎𝑚𝑒𝑡𝑟𝑎𝑙 h𝑃𝑖𝑡𝑐

Page 6: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear DrivesThe pitch circle of a gear is the location on the gear where speed and torque are transmitted.

This happens at the contact point between the gear teeth along a line that passes through the line of centers of the two gears.

Page 7: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear DrivesThe pitch diameter is the diameter of the pitch circle and is used to calculate the speed and torque to the driven shaft.

The pitch circle is only significant in determining the pitch diameter.

Page 8: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Pressure Angle: The angle at which power is passed from the tooth of one gear to the tooth of the other.Commonly pressure angle is 20o

Backlash: the space between the non-driving sides of the adjacent teeth of two meshing gears. Backlash prevents gear binding and wearing.

Root Clearance: the radial distance between the top of the tooth of the pinion and the bottom of the tooth of the gear.

Root Clearance

MECH1200Gear Drives

Page 9: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear DrivesGear Ratio Methods:

Ratio of Pitch Diameters

Ratio of Number of Gear Teeth

Using the manufacturer’s catalog data, you can use either formula because both are listed.

In a plant, you will use teeth because it is easier to count teeth than measure pitch diameter.

Page 10: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Ratio of Pitch Diameters

- Pitch circle diameter of driven (in) - Pitch circle diameter of driver (in)

Page 11: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Ratio of Number of Gear Teeth

- Number of teeth of driven gear - Number of teeth of driver gear

Input A

Output B

10 teeth

28 teeth

11 teeth22 Teeth

𝐺=2211

=2

𝐺=2810

=2.8

Page 12: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Gear ratio determines how fast the driven gear will turn.

The teeth of the driver gear transfer speed to the teeth of the driven gear such that the points on the teeth of the two gears at their pitch diameters move at the same surface speed.

Different size gears have different RPM.

Page 13: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Larger Gear - Torque goes up, speed goes down, by the gear ratio.

Page 14: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Diametral Pitch - The ratio of the number of teeth on the gear to the pitch diameter

It indicates the relative size of the teeth on the gear.

Two gears must have the same diametral pitch in order to mesh

Determine if gears of different diameters or different numbers of teeth have the same size teeth and can mesh properly

Common sizes are 3 - 48 DP

PD –Diametral Pitch – Number of Teeth - Pitch circle diameter (inches)

Gear Module (metric) =

Page 15: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.
Page 16: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

If gears are adjusted so their pitch circles are tangent with each other, the gears will have the correct amount of backlash, therefore the gears are in mesh.

Backlash can be changed by adjusting positions of the gears shaft centers.

As the center distance is increased, the backlash becomes greater.

Page 17: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

PD –Diametral Pitch – Number of Teeth - Pitch circle diameter (inches)

C – Center distance– Pitch Diameter of gear 1 - Pitch Diameter of gear 2

The center distance is the average of the sum of the two pitch diameters.

Backlash =

𝐶=10+13.5

2=11.75

𝑃𝐷=4710

=4.7

0.020-0.030

Page 18: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Backlash is measured in two ways:

1. Direct Dial Indicator Measurement

2. Indirect Dial Indicator Measurement

Page 19: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Direct Dial Indicator Method:

The probe is placed directly on a tooth and oriented perpendicular to the face of the tooth.

The amount of movement of the teeth at or near the pitch circle is the backlash

Page 20: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

MECH1200Gear Drives

Page 21: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Idler Gears

• Idler gears are inserted between input and output gears

• They serve to reverse the direction of rotation of the output gear.

• Idler gears DO NOT affect the gear ratio between the input and output gears.

Idler Gears

Gear Drives MECH1200

Page 22: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

In the figure, gear A is the input, gear B is an idler gear, and gear C is the output:• What is the gear ratio

between gears A and B?• What is the gear ratio

between gears B and C?• What is the mechanical

advantage between A and C?• Which gear supplied the

largest torque?• Which gear rotates at highest

speed? A

B C

13 teeth

21 teeth42 teeth

Gear Drives MECH1200

Page 23: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Gear Types1. Spur Gears

2. Helical Gears

3. Double Helical Gears

4. Herringbone Gears

5. Bevel Gears

6. Worm Gears

Page 24: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Spur Gears

Advantages:

1. Simple construction and installation

2. Low cost

3. Can have:

- external teeth

- internal teeth (such as planetary gears)

- teeth on a rack (rack and pinion)

Disadvantages:

1. Can generate high friction so they are only used for slow to moderate speeds.

2. As velocity increases vibration increases.Images from Wikimedia Commons.

Page 25: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Helical Gears

Advantages:

1. Run more quietly and smoothly than spur gears.

2. More efficient

3. Can run at higher speeds

4. Has a higher load carrying capacity than spur gears of similar size.

Disadvantages:

1. Create thrust loads, thus need to be supported by proper bearings.

2. High cost

Gears on parallel shafts must be of opposite hands.

Image from Wikimedia Commons.

Page 26: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Double Helical and Harringbone Gears

Advantages:

1. Can handle extreme loads

2. The axial thrust associated with single helical gears is cancelled

Disadvantages:

1. High cost

2. Sensitive to misalignment (can bind if not precisely aligned)

Harringbone gears.

Image from Wikimedia Commons.

Page 27: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Bevel Gears Types: Bevel gears can be Straight, Spiral, or Hypoid. The most common is the hypoid, as straight bevel gears generate high noises while at operation.

Advantages:

Operate on shafts intersecting at an angle, usually the angle is 90o (but not always).

Disadvantages:

1. Generally low gear ratios

2. The shaft bearings must support the load.

3. Require complex and precise installation.

Spiral bevel gears Hypoid bevel gears Image from Wikimedia Commons. Miter bevel gears

Page 28: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Worm Gears

Advantages:

1. Run very quietly and smoothly.

2. Resistant to shock loads

3. Can have very high gear ratios in a compact area.

Disadvantages:

1. Low efficiency due to the high friction resulting from a pure sliding action.

2. Worm always should only be the input and the gear should always only be the output.

Images from Wikimedia Commons.

Page 29: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Questions

1. Name four types of gears:Spur gears, helical gears, bevel gears, worm gears.

2. Why a helical gear can carry more load than a spur gear of a similar size can?

Because the contact area between two meshing teeth is larger in helical gears than that in spur gears.

3. True or False: A worm gear may be used to hold a load or to prevent a system from reversing due to its self locking tendency.

False

MECH1200

Page 30: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Open Gear Alignment

Image from Wikimedia Commons.

As a rule of thumb, gears should mesh such that the pitch circles are tangent to each other, they should not be intersecting nor have no point of contact

MECH1200

Page 31: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Gear Wear and Failure

Main causes:

1. Loads: continuous overloads shock loads vibratory loadsHarsh operating environments such as high ambient

temperatures, presence of dust and sand contaminants.

2. Installation: improper installation and alignment.

3. Poor maintenance: improper lubrication level, type, or excess contamination.

MECH1200

Page 32: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Gear Failure Modes

Main causes:

1. Surface Fatigue: Due to repeated surface stresses. Corrective, destructive, or normal pits

2. Wear: Uniform loss of material due to metal to metal contact Depends on lubricant thickness, and gear material.

3. Plastic flow: surface deformation due to soft gear materials or heavy loads.

4. Breakage: due to fatigue stress cycles, shock loads, or severe misalignment.

MECH1200

Page 33: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Enclosed Gear Drives (1)- Gears that are enclosed in a compact sealed case.

- Better known as gearboxes, also called speed reducers. - Advantages: efficient, safe, and compact.

Gearbox Terminology:

1. Shaft Configuration: number and arrangement of shafts. Input shafts and output shafts can be inline or at right-angles.

2. Hand of Drive: the location of the output shaft in relation to the input shaft.

3. Mounting Method: the method by which a gearbox is connected to the prime mover.Common mounting methods: foot mounting, shaft mounting, and scoop mounting.

MECH1200

Page 34: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

34

4. Efficiency: always power out is less than power in due to conversion of some energy to heat.5. Mechanical rating: the maximum power or torque a speed reducer can supply. Safety margin is 200-300% of mechanical rating.6. Thermal rating: the maximum power or torque that can be transmitted continuously. This is determined by the ability of the gearbox unit to dissipate the heat. 7. Horsepower and Torque Capacity: rated input horsepower is the maximum horsepower at the input that can be transmitted safely. Maximum output horsepower is less than the input horsepower.8. Service factor: Oversizing the gearbox for longer service life9. Backstops: mechanisms such as clutches that prevent reverse rotation.

Enclosed Gear Drives (2) MECH1200

Page 35: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

10. Gearbox Ratio:

- - Gearboxes can have single, double, or multiple stages

(reductions) in them.- As number of stages increases, gearbox ratio increases, but

efficiency decreases.

11. Overhung load: must not exceed the maximum load, to avoid loss of service life and broken shafts.

12. Thrust loads: loads that are parallel to the axis of the shaft. Examples: 1) thrust from helical gears.2) Thrusts from external loads such as fans and mixers3) Thrusts resulting from thermal expansion

Enclosed Gear Drives (3) MECH1200

Page 36: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Enclosed Gearbox TypesConcentric Gearboxes: - Also called inline speed reducers- All are foot-mounted- Single to quadruple reductions (stages)- Ratios are typically 1.5:1 to 500:1

Worm Gearboxes:

Advantages: compact, very high ratios (as high as 3600:1), can be mounted in various ways, low cost, different shaft arrangements

Disadvantages: very large amounts of heat are generated.

Parallel shaft Gearboxes: gearing shafts are parallel to each other. Used for very high loads.

MECH1200

Page 37: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Example: Five-Speed Gearbox

1 3 5

2 4 RN3

N7

N8

N11

N2

N12N6

N4

N1

N5

N10

NI

N8

Source: Siemens Technik Akademie Berlin

MECH1200

Page 38: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

- Internal gears are used.- Two types: single stage and

differential planetary gears.

Sun gear

Planet gears

Stationary ring gear

The main mechanism used in planetary gearboxes is the sun and planet gear.

All Pictures from Wikimedia Commons.

Planetary Gearboxes MECH1200

Page 39: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Gearbox Installation and Troubleshooting1. Nameplate: contains rated power and ratio,

and serial number.

2. Modifications: never modify or weld on the case.

3. Foundation: must be solid and rigid to ensure smooth and aligned running.

4. Alignment: must be as precise as possible. Use level, feeler gauges, and shims to make sure the unit is in line with all connected machinery and eliminate soft-foot.

MECH1200

Page 40: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

5. Shaft Connections: Shaft couplings, belts, and chains must be aligned properly. Proper guards should always be placed to avoid injury to personnel and damage to equipment.

6. Lubrication: proper type and quantity of lubricant are needed, and could be found on nameplates. Lubricant temperature and level should be checked regularly.

7. Storage: gearboxes should be stored in rust preventative oil.

Some storage methods:

1) The gearbox unit is completely filled with oil.

2) Keep the operational oil level, but add a rust inhibiting fluid.

3) Completely drain the gearbox of oil, and spray its internal parts with the rust-inhibiting fluid.

Refer to the Troubleshooting Quick Guide on page 420 in text.

MECH1200

Page 41: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Questions1. Mark each statement as true or false:

a) The worm gear is known for its high efficiency

False, the worm gear transmitts power by pure sliding, so its efficiency is low, and results in a lot of heat generation.

b) Double helical gears can transmit high torques but they suffer from very high axial thrust compared to helical gears.

False, axial thrust is cancelled out in double helical gears.

c) Spur gears are simple and have a low cost, but their high axial thrusts make them unattractive.

False, there is no axial thrust in spur gears.

d) Harringbone gears cancel out the overhung load.

False, overhung load is independent of the gear type.

2. What is a speed reducer? A speed reducer is another name for a gearbox, it indicates that the purpose of a gearbox is mainly to multiply torque, which results in speed reduction, and thus is the name speed reducer.

MECH1200

Page 42: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

Questions3. What type of gears is shown in the picture below?

a) Spur gear

b) Worm gear

c) Harringbone gear

d) Rack and pinion

e) Bevel gear

4. Which part is always the input in the gear above?

The worm should always be the input.

5. When compared to the input worm, the output gear ideally has: a) Higher torque, higher speed, and lower power

b) Higher torque, lower speed, and higher power

c) Lower torque, higher speed, and power stays the same

d) Higher torque, lower speed, and power stays the same

Note: Power can never be amplified. Ideally it stays the same, but practically it always reduces due to friction.

MECH1200

Page 43: Gear Drives – Chapter 14 Gears Function of gears – Transmit mechanical power – Change rotational speed – Change torque – Maintain constant speed ratio.

5. What type of gears is shown in the picture below?a) Spur gear

b) Worm gear

c) Harringbone gear

d) Helical gear

e) Bevel gear

6. What is the most common type of bevel gears?Hypoid bevel gears

7. In order for two gears to mesh together they must have:

a. The same module

b. The same root diameter

c. The same size

d. The same number of teeth

e. The same major diameters

Questions MECH1200