Unit 2: Mechanics

TOPIC: MECHANISMS

In this topic, learners are required to be able to:

  1. define mechanisms
  2. differentiate between types of simple mechanisms
  3. make illustrations of the types of mechanisms.
  4. give elaborate examples of how different mechanisms are applied in real life situations
  5. mention the advantages of hydraulics over pneumatics and vice versa.

 

2.1 Definition of Mechanism

In simple terms, we can say a mechanism is a system of parts working together in a machine.

 

2.2 Types of Mechanisms

Mechanisms generally consist of moving components which give the mechanism its name. They can include but not limited to:

  • Levers
  • Gears and gear trains.
  • Belt and chain drives.
  • Cam and followers.
  • Linkage.

Levers

A lever is a rigid body capable of rotating on or move from a point. A lever typically amplifies an input force to provide a greater output force, which is said to provide leverage. The ratio of the output force to the input force is the mechanical advantage of the lever.

 

2.2 Gears and Gear Trains

Gears are toothed, mechanical transmission elements used to transfer motion and power between machine components. Operating in mated pairs, gears mesh their teeth with the teeth of another corresponding gear or toothed component which prevents slippage during the transmission process.

 

Types of gears

Gear Axes Configuration

The axes configuration of a gear refers to the orientation of the axes—along which the gear shafts lay and around which the gears rotate—in relation to each other. There are three principal axes configurations employed by gears:

  • Parallel
  • Intersecting
  • Non-parallel, non-intersecting

 

Parallel Gear Configurations

As indicated by the name, parallel configurations involve gears connected to rotating shafts on parallel axes within the same plane. The rotation of the driving shaft (and the driving gear) is in the opposite direction to that of the driven shaft (and driven gear), and the efficiency of power and motion transmission is typically high. Some of the types of gears which employ parallel configurations include spur gears, helical gears, internal gears, and some variants of rack and pinion gears.

Fig.2.2: Gears with a parallel axes configuration.

Image Credit: Nuno Andre/Shutterstock.com

 

Intersecting Gear Configurations

In intersecting configurations, the gear shafts are on intersecting axes within the same plane. Like the parallel configuration, this configuration generally has high transmission efficiencies. Bevel gears—including miter, straight bevel, and spiral bevel gears—are among the group of gears which employ intersecting configurations. Typical applications for intersecting gear pairs include changing the direction of motion within power transmission systems.

Fig. 2.3: Gears with an intersecting axes configuration.

Image Credit: Jim Francis/Shutterstock.com

 

Non-parallel, Non-intersecting Gear Configurations

Gear pairs with a non-parallel, non-intersecting configuration have shafts existing on axes which cross (i.e., are not parallel) but not on the same plane (i.e., do not intersect). Unlike parallel and intersecting configurations, this configuration generally has low motion and power transmission efficiencies. Some examples of non-parallel, non-intersecting gears include screw gears, worm gears, and hypoid gears.

Fig. 2.3: Gears with a non-parallel, non-intersecting axes configuration.

Table 1 – Characteristics of Gears by Type

Note: If applicable, “A” indicates advantageous characteristics and “D” indicates disadvantageous characteristics
Type of Gear Characteristics
Spur
  • Most common type of gear
  • Circular gear body
  • Straight teeth cut or inserted parallel to the gear’s shaft
  • Used for parallel axes configuration
  • Mated with spur gears, internal gears, or gear racks
  • High precision and efficiency (A)
  • Easy to manufacture (A)
  • Does not produce thrust force (A)
  • Capable of handling high speed and high loads (A)
  • Gear teeth experience high stress due to tooth design (D)
  • Noise production during high speeds (D)
Helical
  • Circular gear body
  • Teeth twisted at an angle around gear body
  • Used for parallel axes configuration
  • Available in right-hand and left-hand designs
  • Available in single and double helical designs
  • Gradual tooth engagement and less impact loading (A)
  • Quieter, smoother operation (A)
  • Capable of handling greater loads (A)
  • Lower efficiency (D)
  • Higher design complexity, greater cost of manufacturing (D)
  • Single helical design products thrust force (D), double helical does not (A)
Bevel
  • Cone-shaped gear body
  • Used for intersecting axes configuration
  • Available in straight, spiral, and Zerol® bevel tooth designs
  • Straight: simplest bevel gear design and easiest to manufacture (A); high impact, noise level, and stress (D)
  • Spiral: gradual tooth engagement and less impact loading, noise, and vibration (A); higher design complexity and greater cost of manufacturing (D)
  • Zerol®: Quieter and smoother than straight bevel, able to rotate in both directions unlike spiral bevel (A)
Worm
  • Pair comprised of a circular gear and a screw-shaped gear
  • Used for non-parallel, non-intersecting axes configuration
  • Large gear ratios and gear reduction (A)
  • Quiet, smooth operation (A)
  • Self-locking mechanism (A)
  • Low transmission efficiency (D)
  • Large amounts of friction (D)
Rack and Pinion
  • Pair comprised of a gear rack and cylindrical gear
  • Used for parallel axes configuration
  • Rack mated with spur or helical gear
  • Converts rotational motion to linear motion or vice versa
  • Simple design, easy to manufacture (A)
  • Capable of handling greater loads (A)
  • Transmission cannot continue infinitely in one direction (D)
  • Large amount of backlash between mated teeth (D)
  • Gear teeth experience high friction and stress due to tooth design (D)

 

For more details on gears and their detailed illustrations, go to https://www.thomasnet.com/articles/machinery-tools-supplies/understanding-gears/

 

2.3 Belt and chain drive

Belt and chain drives are used to transmit power from one rotational drive to another. A belt is a flexible power transmission element that runs tightly on a set of pulleys. A chain drive consists of a series of pin-connected links that run on a set of sprockets.

 

 

Fig. 2.4 Belt and Chain Drives

Calculating the Mechanical Advantage

Mechanical Advantage is the ratio of the existing weight or load to the acting force; or, the ratio of the distance through which the force is exerted to the distance the weight is raised. For example, a machine has a mechanical advantage of 5 if an applied force of 1 kg can counterbalance a weight of 5 kg.  Or in simpler terms the load is divided by the effort.  So the load (or weight)  5kg is divided by the force (or effort) of 1 kg to equal a mechanical advantage of 5.

To work out a velocity ratio the equation is distance moved by effort divided by the distance moved by load.  I can calculate a velocity ratio by looking at pulley systems.  Pulleys are used to change the speed, direction of rotation or the turning force or torque.  A pulley system typically consists of two pulley wheels each on a shaft that will be connected by a belt.  This will transmit rotary motion and force from the input, or driver shaft, to the output or driven shaft.

 

If the pulley wheels are different sizes, the smaller one will spin faster than the larger one. The difference in speed is called the velocity ratio. This is calculated using the formula:

Velocity ratio = diameter of the driven pulley ÷ diameter of the driver pulley

So Velocity ratio = 120mm ÷ 40mm = 3

If the pulley system is a two pulley one or a four pulley one, the distance moved by effort is multiplied by 2 an4 respectively

If you know the velocity ratio and the input speed of a pulley system, you can calculate the output speed using the formula:

Output speed = input speed ÷ velocity ratio

So the output speed = 100rpm ÷ 3 = 33.3 rpm

The velocity ratio of a pulley system also determines the amount of turning force or torque transmitted from the driver pulley to the driven pulley. The formula is:

Output torque = input torque × velocity ratio.

To work out efficiency, you must use the equation: mechanical advantage divided by velocity ratio x 100%

 

 

Application of Chain Drives

Used in automobiles and other vehicles

Used to transmit power to wheels of vehicles

Used in chain pumps

Used in bicycles

Used inside certain motors

 

Application of Belt Drives

Belt drives are used mainly for industrial applications. Some of these include:

Logging machines,

Oil & natural gas,

Mining,

Aggregate conveyors,

Road construction and

Agriculture machines

 

2.4 Cam and followers

A cam is a mechanical device used to transmit motion to a follower by direct contact. The driver is called the cam and the driven member is called the follower. In a cam follower pair, the cam normally rotates while the follower may translate or oscillate. A familiar example is the camshaft of an automobile engine, where the cams drive the push rods (the followers) to open and close the valves in synchronization with the motion of the pistons.

Types of Cams

Types of followers

 

For more details visit: http://engineering.myindialist.com/2013/introduction-of-cams-and-types-of-cam-and-followers/#.XopPpnLLc_4

 

Or: https://aidanross7.wordpress.com/2010/10/10/s13-design-and-analyse-basic-mechanical-systems/

 

 

2.5 Linkages

A mechanical linkage is an assembly of bodies connected to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid. The connections between links are modeled as providing ideal movement, pure rotation or sliding for example, and are called joints. A linkage modeled as a network of rigid links and ideal joints is called a kinematic chain. For more examples and notes please visit the following URLs. https://en.wikipedia.org/wiki/Linkage_(mechanical)

 

For more examples watch these clips

https://www.youtube.com/watch?v=5lGy8mQxZbY

 

 

Examples of linkages include the windshield wiper, the bicycle suspension, the leg mechanism in a walking machine and hydraulic actuators for heavy equipment. In these examples the components in the linkage move in parallel planes and are called planar linkages.

 

2.6 What are Couplings?

Couplings are mechanical devices used to transmit power/torque from one shaft to another shaft.

Power can be transmitted by means of various gear arrangements or drives only if the shafts are parallel. Couplings are used when the shafts are in a straight line and are to be connected end to end to transmit power.

 

General application OR Uses-

  • To transmit power from driver shaft to driven shaft.
  • To connect or couple 2 components which are manufactured separately eg. output motor shaft and generator.
  • To introduce an extra flexibility while transmitting power in case of space restrictions.
  • To introduce protection against overloads.
  • To reduce the transmission of shock loads from one shaft to another by using flexible couplings.

Types of Couplings-

Here are the different types of couplings which are most widely used in industries and machines-

  1. Muff or Sleeve coupling

 

Sleeve couplings are nothing but just sort of thick hollow cylinder/pipe called sleeve or muff. The sleeve is manufactured keeping the diameter of shaft in mind so that the shaft fits perfectly into the sleeve. The driver & driven, both the shafts are then inserted into each side of the sleeve. Also two or more threaded holes are provided into the sleeve as well as in both of the shaft’s end so that they don’t move in longitudinal direction when the bolts are inserted into them. Also the keyway and key ensures that the shaft and sleeve doesn’t slip.
The sleeve coupling is easy to manufacture as there are neither more nor more number of parts.

Application-
They are used where the shafts don’t require any alignments and load capacity is light to medium duty.

  1. Split Muff coupling

Fig. 2.6: Split muff coupling with parts labelled.

In split muff coupling, the sleeve or muff isn’t a single different part instead it is split into 2. The muffs are semi-cylindrical in shape which then fits over the shaft. Threaded holes are provided on the muffs so that both the shafts can be joined with steel bolts or studs.
The special feature of this coupling is that it can be assembled and disassembled without changing the position of shaft.

Application-
They are used for medium to heavy duty load with moderate speed.

  1. Flange coupling

Fig. 2.7: Flange coupling with labeled parts

 

Flange coupling is also an easy to manufacture coupling and is similar to sleeve coupling. Here, there are flanges on either side of the 2 sleeves. Both the flanges consist of equal number of threaded holes for bolting purpose. The flanges are then joined together with bolts and nuts. A key section is also provided on the hub and shafts so that there is no slipping condition.
A tapered key is used here. This ensures that the hub doesn’t loosen up or move backward and stays attached to the shafts.

Application-
Flange couplings are used for medium & heavy-duty industrial applications.

  1. Bush Pin type flexible coupling

 

Fig. 2.8: Bush Pin Type Flexible Coupling with Labeled Parts

You could say that this coupling is an upgraded version of flange coupling. The only difference between them is the usage of rubber bushings. Slightly thick rubber bushings are designed so that the studs or bolts perfectly fit inside it and bushing fits perfectly inside the holes provided.
The major advantage of using this coupling is that it can be used for slightly misaligned shafts. The rubber bushings add a certain amount of flexibility to the coupling which also helps to absorb shocks and vibrations.

Application-
Bush pin type couplings are used where there is a little amount of angular, parallel or axial misalignment. They are used in medium duty applications in electric motors & machines.

  1. Gear coupling

 

Fig. 2.9: Gear coupling with labeled parts

 

The gear coupling is another modified version of the flange coupling. In gear coupling, the flange and hub are different parts assembled together instead of a single part as in flange coupling.
The hubs are externally splined but they are so thick and deep that you can regard them as gear teeth. Also the flanges have internal teeth. The gear ratio is 1:1 and are meshed together. The single joint gear couplings are limited to lower angular misalignments.

Application-
Gear couplings are used for heavy-duty applications where requirement of torque transmission is higher.

 

  1. Universal Coupling

As the name suggests, this type of coupling can be used anywhere. The universal joint can transmit power even at high parallel or angular misalignments. It consists of a pair of hinges close together perpendicular to each other connected by cross shafts. Unlike Rzeppa joint, the universal joint is not a constant velocity joint. This means that rpm of driver and driven shaft won’t be same at every angle.

Application-
Universal joints are used in machines where there are space restrictions or high flexibility is needed.

A good example would be the usage of this joint for transmission of power from engine to the rear differential via propeller shaft. You could clearly see the universal joint under the trucks! And it really feels so fascinating to watch it in running condition 😀

 

Fig. 2.10 The Universal joint

 

 

There are 4 primary types of mechanical motion;

  1. LINEAR – Motion that moves an object in a straight line from its starting point. Newtons 1st law suggests an object will continue to travel in a straight line unless a unbalanced force is applied. Forces such as drag, gravity and friction will prevent this on earth
  2. RECIPROCATING – Motion that pivots back and forth in a linear direction. This could be likened to a saw blade or a pneumatic drill
  3. ROTARY – Motion that turns the object in a complete 360° circle around and axis, often repeatedly. This motion is simple to remember as wheel or fan. The strength of this motion is defined as torque and the repetitions are counted in RPM (Rev. per min)
  4. OSCILLATING – This is a combination of rotary and reciprocating motions. The object is offset from the axis and rotates back and forth in an arc. This is best likened to the swing of a pendulum

 

2.7 Energy

There are 10 main different sources of energy that are used in the world to generate power. While there are other sources being discovered all the time, none of them has reached the stage where they can be used to provide the power to help modern life go on.

All of these different sources of energy are used primarily to produce electricity. The world runs on a series of electrical reactions – whether you are talking about the car you are driving or the light you are turning on. All of these different sources of energy add to the store of electrical power that is then sent out to different locations via high powered lines.

For more information about the types and sources of energy, please visit: https://www.conserve-energy-future.com/different-energy-sources.php

 

2.8 Hydraulics and Pneumatics

 

There are two types of fluid power circuits; Hydraulics and Pneumatics.

Most fluid power circuits use compressed air or hydraulic fluid as their operating media. While these systems are the same in many aspects, they can have very different characteristics in certain ways.

For example: remote outdoor applications may use dry nitrogen gas in place of compressed air to eliminate freezing problems. Readily available nitrogen gas is not hazardous to the atmosphere or humans. Because nitrogen is usually supplied in gas cylinders at high pressure, it has a very low dew point at normal system pressure. The gas may be different but the system’s operating characteristics are the same.

Hydraulic systems may use a variety of fluids — ranging from water (with or without additives) to high-temperature fire-resistant types. Again the fluid is different but the operating characteristics change little.

Pneumatic systems

Most pneumatic circuits run at low power — usually around 2 to 3 horsepower. Two main advantages of air-operated circuits are their low initial cost and design simplicity. Because air systems operate at relatively low pressure, the components can be made of relatively inexpensive material — often by mass production processes such as plastic injection molding, or zinc or aluminum die-casting. Either process cuts secondary machining operations and cost.

First cost of an air circuit may be less than a hydraulic circuit but operating cost can be five to ten times higher. Compressing atmospheric air to a nominal working pressure requires a lot of horsepower. Air motors are one of the most costly components to operate. It takes approximately one horsepower to compress 4cfm of atmospheric air to 100psi. A 1-hp air motor can take up to 60 cfm to operate, so the 1-hp air motor requires (60/4) or 15 compressor horsepower when it runs. Fortunately, an air motor does not have to run continuously but can be cycled as often as needed.

Air-driven machines are usually quieter than their hydraulic counterparts. This is mainly because the power source (the air compressor) is installed remotely from the machine in an enclosure that helps contain its noise.

Because air is compressible, an air-driven actuator cannot hold a load rigidly in place like a hydraulic actuator does. An air-driven device can use a combination of air for power and oil as the driving medium to overcome this problem, but the combination adds cost to the circuit.

Air-operated systems are always cleaner than hydraulic systems because atmospheric air is the force transmitter. Leaks in an air circuit do not cause housekeeping problems, but they are very expensive. It takes approximately 5 compressor horsepower to supply air to a standard hand-held blow-off nozzle and maintain 100 psi. Several data books have charts showing cfm loss through different size orifices at varying pressures. Such charts give an idea of the energy losses due to leaks or bypassing.

Hydraulic systems

A hydraulic system circulates the same fluid repeatedly from a fixed reservoir that is part of the prime mover. The fluid is an almost non-compressible liquid, so the actuators it drives can be controlled to very accurate positions, speeds, or forces. Most hydraulic systems use mineral oil for the operating media but other fluids such as water, ethylene glycol, or synthetic types are not uncommon. Hydraulic systems usually have a dedicated power unit for each machine. Rubber-molding plants depart from this scheme. They usually have a central power unit with pipes running to and from the presses out in the plant. Because these presses require no flow during their long closing times, a single large pump can operate several of them. These hydraulic systems operate more like a compressed-air installation because the power source is in one location.

A few other manufacturers are setting up central power units when the plant has numerous machines that use hydraulics. Some advantages of this arrangement are: greatly reduced noise levels at the machine, the availability of backup pumps to take over if a working pump fails, less total horsepower and flow, and increased uptime of all machines.

Another advantage hydraulic-powered machines has over pneumatic ones is that they operate at higher pressure — typically 1500 to 2500 psi. Higher pressures generate high force from smaller actuators, which means less clutter at the work area.

The main disadvantage of hydraulics is increased first cost because a power unit is part of the machine. If the machine life is longer than two years, the higher initial cost is often offset by lower operating cost due to the much higher efficiency of hydraulics. Another problem area often cited for hydraulics is housekeeping. Leaks caused by poor plumbing practices and lack of pipe supports can be profuse. This can be exaggerated by overheated low-viscosity fluid that results from poor circuit design. With proper plumbing procedures, correct materials, and preventive maintenance, hydraulic leaks can be virtually eliminated.

Another disadvantage could be that hydraulic systems are usually more complex and require maintenance personnel with higher skills. Many companies do not have fluid power engineers or maintenance personnel to handle hydraulic problems.

Typical pneumatic circuit

Figure 5-1 includes a pictorial representation and a schematic drawing of a typical pneumatic circuit. It also has a pictorial and schematic representation of a typical compressor installation to drive the circuit (and other pneumatic machines). Seldom, if ever, is the compressor part of a pneumatic schematic. Power for a typical pneumatic circuit comes from a central compressor facility with plumbing to carry pressurized air through the plant. Pneumatic drops are similar to electrical outlets and are available at many locations.

 

Why schematic drawings?

Schematic drawings make it possible to show circuit functions when using components from different manufacturers. A 4-way valve or other component from one supplier may bear little physical resemblance to one from other suppliers. Using actual cutaway views of valves to show how a machine operates would be fine for one circuit using a single supplier’s valves. However, another machine with different parts would have a completely different-looking drawing. A person trying to work on these different machines would have to know each brand’s ins and outs . . . and how they affect operations. This means designing and troubleshooting every circuit would require special and different knowledge. Using schematic symbols requires learning only one set of information for any component.

Schematic symbols also give more information than a picture of the part. It may almost impossible to tell if a 4-way valve is 3-position by looking at a pictorial representation. On the other hand, its symbol makes all features immediately clear. Another advantage is that by using ISO symbols the drawing can be read by persons from different countries. Any notes or the material list may be unreadable because of language differences, but anyone trained in symbology can follow and understand circuit function. For details of schematic symbols, please visit https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.pinterest.com%2Fpin%2F424112489879674668%2F&psig=AOvVaw0nCg_tnooQW_MWzHUpF715&ust=1586275447939000&source=images&cd=vfe&ved=0CA0QjhxqFwoTCMDJ_OCW1OgCFQAAAAAdAAAAABAK

 

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