Gear (Part I)

Introduction:

In the transmission of motion or power between two shafts, the slipping of a belt or drive is a common phenomenon. The velocity ratio of the system is reduced due to the effect of slipping. In precision machines, in which a definite velocity ratio is of importance (as in watch mechanism), the only positive drive is by gears or toothed wheels. A friction wheel with the teeth cut on it is known as gear or toothed wheel. The usual connection to show the toothed wheels is by their pitch circles. The motion and power transmitted by gears is kinematically equivalent to that transmitted by frictional wheels or discs.

The following are the major pros and cons of the gear drive as compared to other drives, i.e. belt, rope and chain drives:

Advantages

1. It transmits exact velocity ratio.

2. It can be used to transmit large power.

3. It can be used for small centre distances of shafts.

4. High efficiency and has reliable service.

Disadvantages

1. It is costlier than other drives.

2. The error in cutting teeth may cause vibrations and noise during operation.

3. It requires suitable lubricant and reliable method of applying it, for the proper operation of gear drives.

Classification of Gears

1.      According to the position of axes of the shafts.

The axes of the two shafts between which the motion is to be transmitted, may be

(a) Parallel,

(b) Intersecting, and

(c) Non-intersecting and non-parallel.

2.      According to the peripheral velocity of the gears.

The gears, according to the peripheral velocity of the gears, may be classified as :

(a) Low velocity,

 (b) Medium velocity, and

 (c) High velocity.

3.      According to the type of gearing.

The gears, according to the type of gearing, may be classified as :

(a) External gearing,

(b) Internal gearing, and

(c) Rack and pinion.

4.      According to the position of teeth on the gear surface.

 The teeth on the gear surface may be

(a) Straight,

(b) Inclined, and

 (c) Curved.

Terms used in Gears

1. Pitch circle.
It is an imaginary circle which by pure rolling action, would give the same motion as the actual gear.

2. Pitch circle diameter. 
It is the diameter of the pitch circle. The size of the gear is usually specified by the pitch circle diameter. It is also called as pitch diameter.

3. Pitch point. 
It is a common point of contact between two pitch circles.

4. Pitch surface. 
It is the surface of the rolling discs which the meshing gears have replaced at the pitch circle.

5. Pressure angle or angle of obliquity. 
It is the angle between the common normal to two gear teeth at the point of contact and the common tangent at the pitch point. It is usually denoted by φ. The standard pressure angles are 1 14 /2° and 20°.

6. Addendum. 
It is the radial distance of a tooth from the pitch circle to the top of the tooth.

7. Dedendum. 
It is the radial distance of a tooth from the pitch circle to the bottom of the tooth.

8. Addendum circle. 
It is the circle drawn through the top of the teeth and is concentric with the pitch circle.

9. Dedendum circle. 
It is the circle drawn through the bottom of the teeth. It is also called root circle.

Note: Root circle diameter = Pitch circle diameter × cos φ, where φ is the pressure angle.

10. Circular pitch. 
It is the distance measured on the circumference of the pitch circle from a point of one tooth to the corresponding point on the next tooth. 

11. Diametral pitch. 
It is the ratio of number of teeth to the pitch circle diameter in millimetres.

12. Module. 
It is the ratio of the pitch circle diameter in millimetres to the number of teeth. 

13. Clearance. 
It is the radial distance from the top of the tooth to the bottom of the tooth, in a meshing gear. A circle passing through the top of the meshing gear is known as clearance circle.

14. Total depth. 
It is the radial distance between the addendum and the dedendum circle of a gear. It is equal to the sum of the addendum and dedendum.

15. Working depth. 
It is radial distance from the addendum circle to the clearance circle. It is equal to the sum of the addendum of the two meshing gears.

16. Tooth thickness.
It is the width of the tooth measured along the pitch circle.

17. Tooth space. 
It is the width of space between the two adjacent teeth measured along the pitch circle.

18. Backlash. 
It is the difference between the tooth space and the tooth thickness, as measured on the pitch circle.

19. Face of the tooth. 
It is surface of the tooth above the pitch surface.

20. Top land. 
It is the surface of the top of the tooth.

21. Flank of the tooth. 
It is the surface of the tooth below the pitch surface.

22. Face width.
It is the width of the gear tooth measured parallel to its axis.

23. Profile. 
It is the curve formed by the face and flank of the tooth.

24. Fillet radius. 
It is the radius that connects the root circle to the profile of the tooth.

25. Path of contact. 
It is the path traced by the point of contact of two teeth from the beginning to the end of engagement.

26. Length of the path of contact. 
It is the length of the common normal cut-off by the addendum circles of the wheel and pinion.

27. Arc of contact. 
It is the path traced by a point on the pitch circle from the beginning to the end of engagement of a given pair of teeth. The arc of contact consists of two parts, i.e.

(a) Arc of approach. 
It is the portion of the path of contact from the beginning of the engagement to the pitch point.

(b) Arc of recess. 
It is the portion of the path of contact from the pitch point to the end of the engagement of a pair of teeth.

Engine Friction and Lubrication (Part 2)

Introduction:

In previous blog, we have discussed about basic of engine friction and lubrication. Here, we will discuss about various topics like Grading of Lubricating Oil, Grease Lubrication and additives.

 Link to Part 1. Click Here

Grading of Lubricating oil:

Lubricating oils are classified according to viscosity property possessed by them. The Society of Automotive Engineers (SAE), USA, has assigned a unique number code to oil and they are commonly used for grading the lubricating oil. SAE has assigned a number to oil whose viscosity at given temperature falls in certain range. There are two different temperatures used as a reference for assigning unique number code to oils -18° C and 99° C. SAE, 5W, 10W, 20W grades are defined in terms of viscosity at -18° C and they are lubricants which render starting of engine in cold weather easily. While SAE, 20, 30, 40 and 50 grades are defined at 99° C temperature in terms of viscosity and they are lubricants which work satisfactory in normal and hot climatic conditions.

The above mentioned numbers for lubricants are nearly used for their classification according to viscosity and do not indicate the quality of oil since these numbers do not consider some important factors such as, stability, oiliness, etc.

Multigrade oils:

It is possible; with the advent of additives such as viscosity Index improves, to develop lubricants with more than one viscosity at different temperatures. Thus lubricants in SAE-30 grade at 99⁰ C and in the SAE-10 W grade at -18⁰ C oils of this type are known as multgrade oils. Also other possible grades are 5W/20, 20W/20, 20W/40.

There are many advantages of multigrade oils. Some of them are listed below:

·        No need to change oil as per the ambient temperature,

·        Battery life is extended due to ease of cranking even at low ambient temperature,

·        Engine remains healthy for long and required viscosity is maintained under different operating temperatures,

·        It provides easy starting and short warming up period and reduced oil consumption,

·        It retard the build-up carbon inside the combustion chamber and promotes the mileage between the decarbonization,

·         They possess excellent thermal and oxidation stability. Hence, they protect the engine from rusting, corrosion and wear.

Grease Lubrication:

A heavily loaded, low speed and large clearance bearings, thin lubricating oil is not suitable due to high fluidity.

Grease are semi-solid materials manufactured by dispersing a thickening agent in lubricating fluid. The lubricating fluid is generally mineral oil and the thickening agent is usually a metallic soap. The soap dispersed in the oil forms a pattern of minute cris-crossing fibers which trap the oil and prevents the flow. The mineral component provides the lubrication while the soap component gives the body to the greases. The properties of greases mainly depend on the type of soap and lubricating fluids used. Some of the common base greases are:

·        Calcium soap greases,

·        Sodium soap greases,

·        Lithium soap greases,

·        Aluminum soap greases and

·        Non soap greases.

Additives:

Additives are used in greases in the similar manner as in lubricating oils to enhance certain properties for special purpose. The main types of additives used are:

1.     Anti-oxidant

They are used to increase oxidation stability.

2.     Anti-rust

Additives are used to enhance protection against rusting and wear.

3.     Extreme pressure solid additives

They are mainly used to increase the load bearing capacity; molybdenum disulphide is sometimes used to reinforce greases which have to lubricate sliding surfaces for prolonged periods without renewals.

Threaded Fasteners (Screwed Joints)

Threaded Fasteners (Screwed Joints)

Introduction:

This blog is intended to cover the descriptions, uses and materials of threaded fasteners used in practice. Typical methods of fastening or joints use such as devices as bolts, nuts, cap screws, setscrews, rivets, spring retainers, locking devices, pins, keys, welds, and adhesives.

One of the important targets of mechanical design engineers is to reduce the number of fasteners. For example, Jumbo jets such as Boeing’s 747 require as many as 2.5 million fasteners, some of which cost several dollars per piece. To keep cost down, aircraft manufacturers, and their subcontractors, constantly review new fastener design, installation techniques, and tooling.

The helical-thread screw was undoubtedly an extremely important mechanical invention. It is the basis of power screws and threaded fasteners, an important component in nonpermanent joints.

The terminology of screw threads is explained below:

·        The Pitch is the distance between adjacent thread forms measured parallel to the thread axis.

·        The Major diameter is the largest diameter of a screw thread.

·        The Minor or root diameter is the smallest diameter of the screw thread.

·        The Pitch diameter is the imaginary diameter between the major and minor diameters.

·        The Lead is the distance that nut advances parallel to the screw axis when the nut is given one turn. For a single start thread, the lead is the same as the pitch.

Screwed Joints:

A screw thread is formed by cutting a continuous helical groove on a cylindrical surface. Standard screw threads consist of the unified inch series and the metric series. There are two standard profiles in the metric series, both the Unified and merc M threads utlize the same profile, namely:

1.      M

2.      MJ

A screw made by cutting a single helical groove on the cylinder is known as single threaded (or single-start) screw and if a second thread is cut in the space between the grooves of the first, a double threaded (or double-start) screw is formed. Similarly, triple and quadruple (i.e. multiple-start) threads may be formed. The helical grooves may be cut either right hand or left hand commonly known as right handed or left handed screw.

It is mainly composed of two elements i.e. a nut and bolt. The screwed joints are widely used where the machine parts are required to be readily connected or disconnected without damage to the machine or the fastening. Following are the advantages and disadvantages of the screwed joints.

Advantages

1. Screwed joints are highly reliable in operation and easy to operate.

2. Screwed joints are convenient to assemble and disassemble.

3. A wide range of screwed joints may be adapted to various operating conditions.

4. Screws are relatively cheap to produce due to standardization and highly efficient manufacturing processes.

Disadvantages

The main disadvantage of the screwed joints is the stress concentration in the threaded portions which are vulnerable points under variable load conditions.

Stresses in Screwed Fastening due to Static Loading

The following stresses in screwed fastening are induced due to static loading are important from the subject point of view:

1. Internal stresses due to screwing up forces,

2. Stresses due to external forces, and

3. Stress due to combination of stresses.

Stresses due to External Forces

When bolt is subjected to an external load, the following stresses are induced in it:

1.      Tensile stress.

The bolts and screws usually carry a load in the direction of the bolt axis which produces a tensile stress in them.

2.      Shear stress.

Nut and bolts are used to prevent the relative movement of two or more parts, as in case of flange coupling, and then the shear stress is induced in the bolts. When the bolts are subjected to direct shearing loads, they should be located in such a way that the shearing load comes upon the body (i.e. shank) of the bolt and not upon the threaded portion. In some cases, the bolts may be relieved of shear load by using shear pins. When a number of bolts are used to share the shearing load, the finished bolts should be fitted to the reamed holes.

3.      Combined tension and shear stress.

When the bolt is subjected to both tensile and shearing loads, as in case of coupling bolts or bearing, then

·        The diameter of the shank of the bolt is obtained from the shear load and

·        That of threaded part from the tensile load.

A diameter slightly larger than that required for either shear or tension may be assumed and stresses due to combined load should be checked for the following principal stresses. When Factor of safety is taken into account, diameter should be increased.

Manufacturing Processes (Part I)

Introduction:

In engineering practice, the knowledge of manufacturing processes plays an important role for mechanical engineers.  The following are the various manufacturing processes used in Mechanical Engineering.

1. Primary shaping processes.

The processes used for the preliminary shaping of the machine component are known as primary shaping processes. The common operations used for this process are casting, forging, extruding, rolling, drawing, bending, shearing, spinning, powder metal forming, squeezing, etc.

2. Machining processes.

The processes used for giving final shape to the machine component, according to planned dimensions are known as machining processes. The common operations used for this process are turning, planning, shaping, drilling, boring, reaming, sawing, broaching, milling, grinding, etc.

3. Surface finishing processes.

The processes that are used to provide a good surface finish for the machine component are known as surface finishing processes. The common operations used for this process are polishing, buffing, honing, lapping, abrasive belt grinding, barrel tumbling, electroplating, super finishing, etc.

4. Joining processes.

The processes used for joining machine components are known as joining processes. The common operations used for this process are welding, riveting, soldering, brazing, screw fastening, pressing, sintering, etc. Joints can be of following types:

·        Temporary joints,

·        Semi-permanent joints,

·        Permanent joints.

5. Processes effecting change in properties.

These processes are used to impart certain specific properties to the machine components so as to make them suitable for particular operations or uses. Such processes are heat treatment, hot-working, cold-working and shot peening.

Casting

It is one of the most important manufacturing processes used in Mechanical Engineering practice. The castings are obtained by remelting of ingots* in a cupola or some other foundry furnace and then pouring this molten metal into metal or sand moulds.

Engine blocks is major example of casting.

The various important casting processes are as follows:

1.      Sand mould casting.

The casting produced by pouring molten metal in sand mould is called sand mould casting. It is particularly used for parts of larger sizes.

2. Permanent mould casting.

The casting produced by pouring molten metal in a metallic mould is called permanent mould casting. It is used for casting aluminum pistons, electric iron parts, cooking utensils, gears, etc. The permanent mould castings have the following advantages:

(a) It has more favorable fine grained structure,

(b) The dimensions may be obtained with close tolerances,

(c) The holes up to 6.35 mm diameter may be easily cast with metal cores.

3. Slush casting. It is a special application of permanent metal mould casting. This method is used for production of hollow castings without the use of cores.

4. Die casting.

The casting produced by forcing molten metal under pressure into a permanent metal mould (known as die) is called die casting. A die is usually made in two halves and when closed it forms a cavity similar to the casting desired. One half of the die that remains stationary is

known as cover die and the other movable half is called ejector die. The die casting method is mostly used for castings of non-ferrous metals of comparatively low fusion temperature. This process is cheaper and quicker than permanent or sand mould casting. Most of the automobile parts like fuel pump, carburetor bodies, horn, heaters, wipers, brackets, steering wheels, hubs and crank cases are made with this process. Following are the advantages and disadvantages of die casting:

Advantages

(a) The production rate is high, ranging up to 700 castings per hour.

(b) It gives better surface smoothness.

(c) The dimensions may be obtained within tolerances.

(d) The die retains its trueness and life for longer periods. For example, the life of a die for zinc base castings is up to one million castings, for copper base alloys up to 75 000 castings and for aluminum base alloys up to 500 000 castings.

(e) It requires less floor area for equivalent production by other casting methods.

(f) By die casting, thin and complex shapes can be easily produced.

(g) The holes up to 0.8 mm can be cast.

Disadvantages

(a) The die casting units are costly.

(b) Only non-ferrous alloys are casted more economically.

(c) It requires special skill for maintenance and operation of a die casting machine.

5. Centrifugal casting. The casting produced by a process in which molten metal is poured and allowed to solidify while the mould is kept revolving, is known as centrifugal casting. The metal thus poured is subjected to centrifugal force due to which it flows in the mould cavities. This results in the production of high density castings with promoted directional solidification. The examples of centrifugal castings are pipes, cylinder liners and sleeves, rolls, bushes, bearings, gears, flywheels, gun barrels, piston rings, brake drums, etc.

Casting Design

An engineer must know how to design the castings so that they can effectively and efficiently render the desired service and can be produced easily and economically. In order to design a casting, the following factors must be taken into consideration:

1. The function to be performed by the casting,

2. Soundness of the casting,

3. Strength of the casting,

4. Ease in its production,

5. Consideration for safety, and

6. Economy in production.

In order to meet these requirements, a design engineer should have a thorough knowledge of production methods including pattern making, moulding, core making, melting and pouring, etc. The best designs will be achieved only when one is able to make a proper selection out of the various available methods. However, a few rules for designing castings are given below to serve as a guide:

1. The sharp corners and frequent use of fillets should be avoided in order to avoid concentration of stresses.

2. All sections in a casting should be designed of uniform thickness, as far as possible. If, however, variation is unavoidable, it should be done gradually.

3. An abrupt change of an extremely thick section into a very thin section should always be avoided.

4. The casting should be designed as simple as possible, but with a good appearance.

5. Large flat surfaces on the casting should be avoided because it is difficult to obtain true surfaces on large castings.

6. In designing a casting, the various allowances must be provided in making a pattern.

7. The ability to withstand contraction stresses of some members of the casting may be improved by providing the curved shapes e.g., the arms of pulleys and wheels.

8. The stiffening members such as webs and ribs used on a casting should be minimum possible in number, as they may give rise to various defects like hot tears and shrinkage, etc.

9. The casting should be designed in such a way that it will require a simpler pattern and its moulding is easier.

10. In order to design cores for casting, due consideration should be given to provide them adequate support in the mould.

11. The deep and narrow pockets in the casting should invariably be avoided to reduce cleaning costs.

12. The use of metal inserts in the casting should be kept minimum.

13. The markings such as names or numbers, etc., should never be provided on vertical surfaces because they provide a hindrance in the withdrawal of pattern.

14. A tolerance of ± 1.6 mm on small castings (below 300 mm) should be provided. In case more dimensional accuracy is desired, a tolerance of ± 0.8 mm may be provided.

In next blog, more topics like forging processes will be discussed.

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