VALVE ROTATORS

VALVE ROTATOR

A valve rotator turns the valve on its movement, due to this service life of valve increase, it prevents the carbon deposition on the seat and valve guide, and also it helps to maintain the valve at uniform temperature.and prevent pitting due to hotspots  As the valve rotates, the seating surfaces are constantly changing. This helps prevent forming deposits and keeps the valve from developing hot spots.

TYPES OF VALVE ROTATOR

There are two types of valve rotators

1.non- positive type or release type

2. positive type

NON- POSITIVE TYPE

Non positive type rotators release the spring tension from the valve while open. this allows the valve to rotate from engine vibration

There are two types of non positive valve rotators

1.split-collet valve rotators

2. thimble valve rotators

SPLIT-COLLET VALVE ROTATORS

 The stem of the valve has three annular grooves of semi-circular section and the inner surface of each collet has three semicircular ribs. These press against each other providing a small radial clearance between the valve and the collects. The vibration of the valve gear rotates the valve at between 15 and 25 rpm at engine speeds above 1500 rpm.

 THIMBLE VALVE-ROTATOR.

In the thimble rotator design, a steel cap fits over the end of the valve stem and rests on two semicircular collets which fit in the valve-stem groove below. The valve spring maintains a pressure on the retainer against these collets and so keeps the valve closed. When the valve is required to be opened, the rocker-arm presses down the cap which in turn bears against the two collets, and then moves the valve spring and retainer downwards. The spring pressure is now taken by the cap, and hence the valve is freed from spring pressure. It is still moved
downwards, because the closed end of the cap then abuts the valve-stem end, but it is free to turn.

POSITIVE TYPE VALVE-ROTATORS ALSO KNOWN AS ROTOCAP

 

This incorporates a ball-retaining plate with six ramped grooves for balls to roll along. A small spring pushes each of these balls to one side. A Belleville-type dished spring washer fits over these balls to form an upper race, which is supported on its outer edge by a spring-seat retainer. This retainer holds the whole assembly together and also provides a seat for the helical-coil valve springs. In the closed position of the valve, the dished washer is suspended between the spring-seat retainer and the ball-retainer, so that the balls move freely to the top of the ramp and abut against the end of the groove. During opening of the valve, the dished spring washer deflects with the increase in the compression load on the valve spring. The outer edge of the dished washer bears against the spring-seat retainer as before, but the inner part of the washer now bears against the six balls and hence pushes them down their ramps. The ramps are so shaped that, as contact with the washer is maintained, the spring-seat retainer is rotated and hence the poppet-valve is rotated by the same amount. As the valve closes, the washer comes back to its original position between the spring-seat retainer and the ball-retainer. This releases the load on the balls due to which the small bias springs now push the balls up their ramps and thereby bring the spring-seat retainer and the valve assembly back to its starting position.

 

WINGED VALVE ROTATOR

In two stroke engines valve  operated by hydraulically uses winged valve rotators in this type valve spindle is fitted with wings (vanes) the kinetic energy of the exhaust gas rotates the valve at small amount as it passes

BOLLARD PULL

BOLLARD PULL

BOLLARD PULL

Bollard pull is the static force exerted by a ship on a fixed tow line at zero speed. Trawl pull is a force exerted by a ship on a tow line at speed in another words Bollard pull is the maximum pulling capacity of a tug that can exert

WHY IN BOLLARD PULL NOT BHP

the BHP of the tug is not sufficient to show the efficiency because of loss in transmission, propulsion efficiency and propulsion type Whereas  Bollard pull shows the  efficiency of the maximum pulling capacity of a tug that can exert

TEST

bollard pull test is carried out by dynamometer comprised of load cell,load indicator.

Loadcell is the force measuring link and it is made as part of the rope, connected to the towing hook

• Sustained Bollard Pull is the mean value of the pull during a specific time period (5 to 10 min.). If the measurements are taken every 30 sec., the sustained Bp is the mean value of the readings.
• Maximum Static Bollard Pull is the highest 30 second value read during the test. If the measurements are taken every 30 sec., the maximum static Bp is the highest mean value of two consecutive readings.
• Maximum Bollard Pull is the highest single value measured.

REQUIREMENTS FOR THE BOLLARDPULL TEST

• Test is carried out at slackwater   
  
• Trial site: should allow the use of a long enough towing line and be free of wind, waves, and be located in very deep water with no tides or currents.
• The towline length should be enough to allow free water astern of the vessel. (200 meters minimum recommended). 
• The tug has to be loaded to its designed waterline, in order to ensure the adequate immersion of the propellers. 
• Wind speed during the test should not exceed 5m/s. 
• The trial should be done with no currents if possible; if there’s a current, it should not exceed 1 knot. 
• Measurement should be taken using a calibrated dynamometer or a mechanical load gauge. 
• Steering during the trials should be avoided as much as possible. 
• The readings on the dynamometer should be continuously recorded by a computer, or at 30 sec intervals if a mechanical load gauge is used. 
• The maximum acceptable deviation of the dynamometer is +-2%. 
• Communication between shore and tug should be excellent, for an accurate recording of data relating the bollard pull to the rpm.
• Bollard and the towing hook should be at the same height

TAPPET CLEARANCE

TAPPET CLEARANCE
this is the clearance available between the rocker arm tip and valve stem when the valves are closed position when engine at cooled condition,ie.at the compression stroke
WHY TAPPET CLEARANCE
tappet clearances are necessary to allow for thermal expansion of the valve spindle length at working temperature
ADJUSTING THE CLEARANCE
Bring the piston to TDC of compression stroke, this will ensure by the flywheel marking or fuel pump mark or by checking the camshaft fuel or valve cam position on this position fuel cam lobe will be touch the fuel   

pump or valve cams will be on base position(off cam) that means both valves are closed position,at this time push rod will be free this can be check by rotating push rod. and take the clearance between stem and rocker arm tip by using a feeler gauge and adjusted by loosening or tightening the tappet adjusting screw

EXCESSIVE TAPPET CLEARANCE will cause the valve to open late and close early in the cycle and will reduce maximum lift it will also noise and get damage from the impact of working surface

LESS CLEARANCE will cause the valve to open early and close late with increased maximum lift it may prevent the valve from closing completely as it expands this may cause valve burning and low compression

 

VIBRATIONS

VIBRATION

            If a force displaces an elastic body from its position, the body will develop a restoring force which tends to return it to that state. therefore when the displacing force is removed, the body will move towards the original position Owing to its mass or rather, to its inertia the body on its return movement will not stop at its previous position of equilibrium but will be carried beyond causing a displacement in the opposite direction. the restoring force thus developed in the opposite direction will reverse the action and the process will continue until these consecutive movements called oscillations or vibrations.

BASIC CONCEPT OF VIBRATION

All bodies having mass and elasticity are capable of vibration. When external force is applied on the body, the internal forces are

set up in the body which tend to bring the body in the original position. The internal forces which are set up are the elastic forces which tend to bring the body in the equilibrium position.

MAIN CAUSES OF VIBRATION

 The main causes of vibration are:

1. Unbalanced centrifugal force in the. System due to faulty design and poor manufacturing.

2. Elastic nature of system.

3. External excitation applied on the system

AMPLITUDE

The amplitude of the oscillation is the maximum distance that the oscillating object moves away from the equilibrium position

FREQUENCY OF VIBRATION

            The number of vibrations per second or the rate at which they occur is known as the frequency of vibration

NATURAL FREQUENCY OF VIBRATION

If an elastic body is allowed to vibrate freely it will do so at a certain rake known as its natural frequency of vibration

FORCED VIBRATION

            When displacing force act repeatedly on an elastic body they cause vibrations known as forced vibrations

DAMPING

 It is resistance provided to the vibrating body and vibrations related to it are called damped vibration.

HARMONIC VIBRATION

        When the forced vibration occur at the same rate as the natural frequency of vibration of body or some multiple of it called a harmonic vibration

RESONANCE

            When displacing forces acts repeatedly on an elastic body they cause vibrations known as forced vibrations. when these forced vibrations occur at the same rate as the natural frequency of vibration of the body or some multiple of it called harmonic vibration the free vibration will be reinforced or amplified by forced vibration this condition is known as resonance

VIBRATION ANALYSIS

            The most commonly used method for rotating machines is called vibration analysis. Measurements can be taken on machine bearing casings with seismic or piezo-electric transducers to measure the casing vibrations, and on the vast majority of critical machines, with eddy-current transducers that directly observe the rotating shafts to measure the radial (and axial) vibration of the shaft. The level of vibration can be compared with historical baseline values such as former startups and shutdowns, and in some cases established standards such as load changes, to assess the severity.

            In condition monitoring systems the vibration analysis method is vastly using

 ELIMINATION OF UNNECESSARY VIBRATIONS

 Unwanted vibrations can be reduced by:

1. Removing external excitation if possible.

2. Using shock absorbers.

3. Dynamic absorbers.

4. Proper balancing of rotating parts.

5. Removing manufacturing defects and material inhomogeneities.

6. Resting the system on proper vibration isolators.

WHAT IS THE IMPORTANCE OF VIBRATION STUDY

 Importance  of vibration study. The imp of vibration study is to reduce or eliminate vibration effects over mechanical components by designing them suitably. Proper design and manufacture of parts will reduce. Unbalance  in engines which causes excessive and  unpleasant  stress in rotating system because of vibration, roper design of machine parts will reduce and tear due to vibration and loosening parts. The proper designing and material distribution prevent the locomotive m leaving the track due to excessive vibration which may result in accident or heavy loss. Proper designing of structure buildings can prevent the condition of resonance which causes dangerously large oscillations which may result in failure of that part.