The hot topic among marine engineering students and seafarer preparing for the exam has been of Tappet Clearance. The clearance in between the valve stem and rocker arm is termed as tappet clearance.
They facilitate the operation of rocker arm in a four-stroke engine; by providing sufficient space for thermal expansion.
Tappet clearance is taken when the engine cools down and both exhaust and inlet valve under closed position i.e compression stroke. It is also sometimes being referred to as valve lash in some books.
Q. Why are we require to measure tappet clearance?
There is no real mechanical linkage between the valves and the rocker arm in four-stroke engines. When you open the cover of the cylinder head for inspection, you will find a gap/clearance in between.
There is no physical connection; valves are operated by the movement of push rod operating ( pushing ) rocker arm in one or another direction.
When there is a change i.e “increase or decrease” in this clearance due to thermal or mechanical expansion; valve timing is adversely affected so are the power cycle.
An increase in clearance will lead to late operation and early closure of exhaust and inlet valves. A valve that will open late and close early will have adverse effects on the engine; such as loss of power, improper combustion, hammering, high exhaust temperature, and engine failure.
On another hand, a decrease in tappet clearance will lead to early operation and late closure of valves.
A valve that will open early and close late will have a negative impact on the performance of the engine; such as power loss, leakage of scavenging air, high fuel consumption, heavy shoot formation, loss of compression pressure, and even burning or total valve failure.
So tappet clearance is regularly measured and adjusted based on manufacturer data to ensure the positive operation of inlet and exhaust valves.
In general, the clearance taken is between 0.2mm to 0.6mm depending upon inlet and exhaust valves and engine manufacturers.
Q. How are Tappet Clearance Taken?
Taking tappet clearance for an engine unit is an easy but skillful task. Ensure that the unit is on the TDC, turning the turning gear clockwise up-to-the appropriate markings. Re-insure about the position by checking the fuel cam as well as the freeness of pushrod.
The engine unit is kept at TDC or compression stroke to ensure the availability of both exhaust and inlet valves for the measurement of clearance.
Checking freeness of push rod ensures that the unit is on the compression stroke and not on power stroke; as in the four-stroke engine, two units are at TDC at any given time. Now, once you are ready to measure the tappet clearance; go through your manufacturer manual to ensure the correct clearance settings.
Try to insert a filler gauge of different sizes in between the gap of the valve stem and rocker arm. Select a different set of filler gauge until you find the right one; which just passes through the tappet clearance without much resistance.
A gauge that can’t pass is just too big but no resistance means the size of the gauge is small so try another one. Once you find the right gauge; look for the dimension marked on it. Now compare it with the manufacturer recommendations; Adjust if necessary.
Adjusting Tappet Clearance
Now after correct measurement of tappet clearance; you know whether to increase or decrease to set it at manufacturer recommendation. All you need is to adjust the clearance; but how?
Tappet clearance is adjusted after opening the lock nut above the rocker arm. This lock nut restricts the movement of tappet to ensure proper clearance setting. Open the nut using spanner of appropriate size ( Normally Ring Spanner are Used).
Once the locknut is disengaged; tappet is free to move up and down. Adjust the tappet clearance by adjusting the position of the tappet. To ensure the correct clearance setting; it is a common practice to place the filler gauge of the desired dimension in the clearance area during adjustments.
Make sure you can slide the filler gauge smoothly once it is set; Too easy to pass is a clear indication of high clearance. This is sometimes hard to judge making it a job of skill and experience.
A small turn will ensure movement up to one-hundredth of a millimeter. Place the lock nut in place once done; re-tighten it without affecting the new clearance setting by the accidental movement of the tappet.
Recheck the clearance if necessary and made a record of maintenance for future purposes. Make sure all is right otherwise any misadjustment can lead to severe conditions such as push rod bending or valve failure.
Before boxing up also check :
- Deposit of sludge around the valve spring.
- Condition of lubrication for the rocker arms and pushrod.
- Condition of lubrication for cotter and also for deposited sludge.
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