Introduction to Cams

A Cam is a mechanical element used to drive another element, a follower, through a specified motion by direct contact. A cam converts an input motion (usually rotary motion) into either reciprocating or oscillatory output action. A cam system consists of the cam itself and the follower, which provides the output motion.

Cam devices are versatile, and almost any arbitrarily-specified motion can be obtained. In some instances, they offer the simplest and most compact way to transform motions. The problem in cam design is how to determine a cam contour that will ultimately deliver a specified motion with acceptable velocity, acceleration and jerk. In all cam systems it is important that the follower is always in contact and following the motion of the cam. This is achieved a number of ways including gravity, using a mechanical constraint system (i.e groove), using a spring force and using a pneumatic or hydraulic force.

Classification of Cam Mechanisms:

Cam mechanisms can be classified by:

              a. Modes of input/output motion.                        

              b. The type and the arrangement of the follower.

              c. The shape of the cam.

a. Modes of input/output motion:

The most common modes of input/output motion of cam follower system are:

                              • Rotating cam translating follower.

                              • Rotating follower.

                              • Translating Cam-translating follower .

                              • Stationary Cam-rotating follower.

b. The type and the arrangement of the follower:

                 • Knife-edge            • Flat-face

                 • Roller                    • Spherical-face

The follower can be arranged in two different configurations; in-line follower and offset follower. In in-line follower, the centerline of the follower passes through the centerline of the camshaft. On the other hand, in Offset followers, the centerline of the follower does not pass through the centerline of the camshaft. These two configurations are shown in the figures below.

c. The shape of the cam:

A few commonly used cam shapes are illustrated in the following figures.

Cam Terminology :

Trace point: A theoretical point on the follower corresponding to the point of a fictitious knife-edge follower. It is used to generate the pitch curve. In the case of a roller follower the trace point is at the center of the roller.

Pitch curve: The path generated by the trace point at the follower is rotated about a stationary cam.

Working curve: The working surface of a cam in contact with the follower. For the knife edge follower of the plate cam, the pitch curve and the working curves coincide. In a close or grooved cam there is an inner profile and an outer working curve.

Pitch circle: A circle from the cam center through the pitch point. The pitch circle radius is used to calculate a cam of minimum size for a given pressure angle.

Prime circle (reference circle): The smallest circle from the cam center through the pitch curve.

Base circle: The smallest circle from the cam center through the cam profile curve.

Stroke or throw: The greatest distance or angle through which the follower moves or rotates.

Follower displacement: The position of the follower from a specific zero or rest position (usually it's the position when the follower contacts with the base circle of the cam) in relation to time or the rotary angle of the cam.

Pressure angle: The angle at any point between the normal to the pitch curve and the instantaneous direction of the follower motion. This angle is important in cam design because it represents the steepness of the cam profile.

Next Time we will illustrate the Design of Cam. so keep up with us.

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Types of Bearings

Bearings are used as a mechanical component to transfer the power and to move a certain part, and this is done by utilizing the small frictional force of the bearings, which makes them rotate easily (or move in one direction easily), all the while withstanding the force and weight load acting against them.

Broad classification of bearings can be done according to the principle of operation and the motions they allow. Different types of bearings are used for different purposes.

Some common types of bearings are:

                     

                     1.ball bearings:

                                             a.Deep groove ball bearing.

                                             b.Angular contact ball bearing.

                     2.roller bearings:

                                             a.Needle Roller Bearing.

                                             b.Tapered Roller Bearing

                                             c.Spherical Roller Bearings 

                     3.thrust bearings:

                                             a.Thrust ball bearing.

                                             b.Thrust roller bearing.

1.Ball Bearings 

Ball bearings are the most common and most used type of bearing found in a number of objects. Also known as anti-friction bearings, ball bearings are small metallic or ceramic spheres used to reduce friction between axles and shafts in numerous applications.

These bearings are able to handle both thrust and radial loads, and are used for applications where the load is relatively small. Load in a ball bearing is transmitted from the outer surface to the ball, and from the ball to the inner surface.

Since the ball is spherical in shape, it contacts the inner and outer race at a very small point, which helps it spin evenly and smoothly. But the contact area holding that load is very small, so if the bearing is overloaded then the balls can deform, ruining the bearing. Ball bearings are often used in individual cages to reduce friction in axle assemblies or in a series to absorb the weight placed on a moving part. 

In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it spin very smoothly. But it also means that there is not very much contact area holding that load, so if the bearing is overloaded, the balls can deform or squish, ruining the bearing.

2.Roller Bearings

Roller bearings use cylinders instead of spheres that means the contact between the inner and outer race is a line not a point. Thus the load is spread over a larger area enabling the bearing to handle greater loads than ball bearings. Roller bearings are used in conveyer belt rollers where they hold heavy radial loads, however these bearings are not designed to handle much thrust loading. 

Roller bearings are found in different varieties, such as: (spherical roller bearings - needle roller bearings - cylindrical roller bearing - tapered roller bearing).

2.a. Needle Roller Bearing

Needle roller bearings are special roller bearings having slender cylindrical rollers that enable them to bear highest load capacity for a given radial space of all rolling-element bearings. Needle roller bearings are ideal for applications where high radial load carrying capacity is required but radial space is limited. Such bearings also suit applications where high rotational speed is involved but cannot accommodate axial loads. 

Compared to the ball bearings, needle bearing have a large surface area that is in contact with the bearing outer surfaces. Additionally they are more compact because there is less difference between the diameter of the shaft and the diameter of the bearing. Thrust needle bearings are flat and use a radial pattern of needles while radial needle bearings are cylindrical and use rollers parallel to the axis of the shaft. 

Needle bearings are primarily used in engine components like pumps, compressors, rocker arm pivots, and transmissions. 

2.b. Tapered Roller Bearing

Tapered roller bearings use conical rollers that run on conical races. Both the inner and outer raceways are segments of cones and the rollers are also made with a taper. Unlike other roller bearings, they support both radial and axial loads, and are able to carry higher loads.

The conical geometry of tapered roller bearings provide a larger contact patch, which allows greater loads to be carried as compared to spherical (ball) bearings. The geometry ensures that the tangential speeds of the surfaces of each of the rollers are same as their raceways along the whole length of the contact patch and no differential scrubbing occurs.

This greatly reduces rolling friction and avoids rapid wear. Taper roller bearings are used in the wheel bearings of most trucks, buses, cars, and so on. Due to manufacturing complexities, tapered roller bearings are generally more expensive than ball bearings. 

2.c. Spherical Roller Bearings 

   

Spherical Roller Bearing Rollers used in spherical roller bearings are thicker in the middle and narrow at the ends, and its race is shaped to match. They can adjust to support misaligned loads. 

The construction of spherical rollers is complex and difficult to produce so they are expensive. Apart from that these bearings have higher friction compared to ball bearing because different parts of the spherical rollers run at different speeds on the rounded race. Thus there are opposing forces along the bearing/race contact increasing the friction. 

Spherical bearings are used in numerous applications where rotational motion changes the alignment of its axis of rotation. One of its important Example is a tie rod on a vehicle suspension. Other important uses of spherical bearings have been in car suspensions, trackballs, computer mouse, heavy machinery, sewing machines, drive shafts, etc. 

3.Thrust Bearings

Thrust Bearing Thrust bearings are particular type of rotary bearing that allow rotation between parts used but they are designed to support axial loads like vertical shafts for which spherical, conical or cylindrical rollers are used. They are used in gearsets like in car transmissions between gears and between the housing and the rotating shafts. 

Thrust bearings are of different varieties.

a.Thrust Ball bearings are composed of ball bearings supported in a ring. They are used in low thrust applications where the radial load is very small.

b.Thrust roller bearings are made of small tapered rollers arranged so that their axes converge at a point on the axis of the bearing.

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Types of Valves

Many different types of valves are used in industrial applications worldwide. a valve can be defined as:

“A generic name for a device with a moveable feature that opens and closes a passageway in order to allow, prevent or control the flow of fluids.”

Valves can be categorized into the following basic types:

(Gate valves - Globe valves - Ball valves - Plug valves - Diaphragm valves - Butterfly valve - Needle valves - Check valves - Relief and safety valves - Clapper valve - Thermal expansion valve - Sampling valves)

1.Gate valves

Gate valves are used when a straight-line flow of fluid and minimum restriction is desired. Gate valves are so named because the part that either stops or allows flow through the valve acts somewhat like the opening or closing of a gate and is called, appropriately, the gate. The gate is usually wedge shaped. When the valve is wide open, the gate is fully drawn up into the valve, leaving an opening for flow through the valve the same size as the pipe in which the valve is installed.

Therefore, there is little pressure drop or flow restriction through the valve. Gate valves are not suitable for throttling purposes since the control of flow would be difficult due to valve design and since the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized, gate valves should not be used for throttling.

- Advantages:

                           - No resistance to flow when open

                           - Little pressure drop

                           - Good sealing when closed

                           - Little/no leakage

- Disadvantages:

                           - Flow changes nonlinearly with stem travel

                           - Vibration/cavitation when partially open

                           - Subject to wear

                           - Repair work is difficult

2.Globe valves

Globe valves are probably the most common valves in existence. The globe valve derives its name from the globular shape of the valve body. However, positive identification of a globe valve must be made internally because other valve types may have globular appearing bodies. Globe valve inlet and outlet openings are arranged in several ways to suit varying  requirements of flow.

- Advantages:

Throttling and - regulating flow - Less seat leakage than Gate Valve.

- Disadvantages:

- High head-loss due to flow direction changes

- Dynamics can create pulsation and damage trim/packing/actuators

- Noisy in high pressure applications

- Valves can be very heavy/large in size for a given application.

3.Ball valves

Ball valves, as the name implies, are stop valves that use a ball to stop or start the flow of fluid. The ball performs the same function as the disk in the globe valve. When the valve handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the valve is shut, which requires only a 90-degree rotation of the hand-wheel for most valves, the ball is rotated so the hole is perpendicular to the flow openings of the valve body, and flow is stopped.

Most ball valves are of the quick-acting type (requiring only a 90-degree turn to operate the valve either completely open or closed), but many are planetary gear operated. This type of gearing allows the use of a relatively small hand-wheel and operating force to operate a fairly large valve. The gearing does, however, increase the operating time for the valve. Some ball valves contain a swing check located within the ball to give the valve a check valve feature.

Ball valves are normally found in the following systems aboard ship: seawater, sanitary, trim and drain, air, hydraulic, and oil transfer.

Advantages:

                      - Less expensive

                      - Low maintenance costs

                      - Low torque

                      - Quick action on/off

                      - Compact

                      - No lubrication

                      - Tight sealing

Disadvantages:

                      - Relatively poor for throttling

                      - Throttling leads to seat erosion


4.Butterfly valve

The butterfly valve  may be used in a variety of systems aboard ship. These valves can be used effectively in freshwater, saltwater, and chill water systems aboard ship. The butterfly valve is light in weight, relatively small, relatively quick-acting, provides positive shut-off, and can be used for throttling.

The butterfly valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle. The resilient seat is under compression when it is mounted in the valve body, thus making a seal around the periphery of the disk and both upper and lower points where the stem passes through the seat. Packing is provided to form a positive seal around the stem for added protection in case the seal formed by the seat should become damaged.

To close or open a butterfly valve, turn the handle only one quarter turn to rotate the disk 90°. Some larger butterfly valves may have a hand-wheel that operates through a gearing arrangement to operate the valve. This method is used especially where space limitation prevents use of a long handle.

Butterfly valves are relatively easy to maintain. The resilient seat is held in place by mechanical means, and neither bonding nor cementing is necessary, Because the seat is replaceable, the valve seat does not require lapping, grinding, or machine work.

Advantages:
                      - On/off as well as throttle/regulate
                      - Easily/quickly operated
                      - Good for large flow/low pressure applications due to saving in weight/size/cost
                      - Good for slurries/suspended solids.


5.Relief and safety valves

Safety Valve is a one type of valve that automatically actuates when the pressure of inlet side of the valve increases to a predetermined pressure, to open the valve disc and discharge the fluid ( steam or gas ) ; and when the pressure decreases to the prescribed value, to close the valve disc again. Safety valve is so-called a final safety device which controls the pressure and discharges certain amount of fluid by itself without any electric power support.
Safety Valve is mainly installed in a chemical plant, electric power boiler, gas storage tank, preventing the pressure vessels from exploding or damaging.

Function of the safety valve :
1. "Nozzle" inside the Safety Valve starts to receive a higher pressure from the inlet side of the valve.
2. When the pressure becomes higher than the set pressure, "Disc" starts to lift and discharge the fluid.
3. When the pressure decreases until the predetermined pressure, the force of the spring closes "Disc".
Role of the spare parts
Nozzle --- Pressure Entrance 
Disc ----- Lid
Spring --- Pressure Controller


6.Needle valves

Needle valves have a long, tapered, needle-like point that is used to make relatively fine adjustments in the amount of fluid flow. They are sometimes used as component parts for other valves because the needle allows for a gradual change in the size of the fluid flow opening. It’s Good for fine adjustment throttling.


7.Check valves

Check valves are used in systems employing gate valves because they prevent the reversal of flow in the piping system and there is a low pressure drop across the valve. The pressure of the fluid through the system opens the system, while the weight of the check mechanism will close the valve if the flow is reversed. It’s main function to Prevent reverse flow.



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