The question of how much extra oil should be used in a pneumatic speargun is sometimes queried by owners who use a longer pneumatic gun, but do not have a table value showing the recommended oil volume for that particular length of gun. However they do know what the oil quantity is for a shorter version of the same gun.
The manufacturer of a gun determines the amount of oil that is free to move around inside the gun after all the internal surfaces are fully coated, hence longer guns require more oil to be added and the values required are shown in tables. If a pneumatic gun remains stationary in one position for any length of time then the oil inside the gun drains to the lowest point in the gun leaving only the thinnest of films coating all the other surfaces, the thickness of this residual film being determined by the viscosity of the oil, the ambient temperature and any surface tension effects on the oil wetted parts. What we are interested in is the gun under usage conditions in which case the oil is being spread around inside the gun and coating of the walls is likely to be more uniform after the rest of the oil pools in the lower sections of the gun. That oil coating, like a film of paint, does nothing for gun lubrication during actual gun operation where we want oil to move to the sliding and load bearing surfaces, that oil coming from the "pooled oil" which is free to slosh around the gun's interior.
Any extra length in a (rear handle) pneumatic gun involves a longer inner barrel tube and a longer outer tank tube, so the incremental amount of oil required will be wetting that additional surface area on the tank and inner barrel walls. This additional surface area "Delta S" is easily calculated as (Dit + Dib + Dob) x pi x "Delta L"; where Dit is the internal diameter of the tank tube, Dib is the internal diameter of the inner barrel tube and Dob is the outer diameter of the inner barrel tube. "Delta L" is the additional length added to the gun and in the examples being looked at here I will assume that "Delta L" is equal to 10 cm.
Now for a Mares "Sten" Dit = 3.8 cm, Dib = 1.3 cm and Dob = 1.8 cm
Hence for a 10 cm increase in gun length "Delta S" = (3.8 + 1.3 + 1.8) x pi x 10 = 216.77 sq cm.
If we say that an oil film of 0.5 mm or 0.05 cm covers this extra surface area then the volume is 216.77 x 0.05 = 10.84 cubic centimetre (cc) or 10.84 ml. Thus for an oil film of half that amount or 0.025 cm the extra volume required will be 5.42 ml. Note that we are neglecting the annular shape of the oil film itself in calculating the additional oil volume, but the result will be close enough for our purposes.
Another example is the diminutive "Taimen" where Dit = 2.6 cm, Dib = 1.0 cm and Dob = 1.1 cm
Hence for a 10 cm increase in gun length "Delta S" = (2.6 + 1.0 + 1.1) x pi x 10 = 147.65 sq cm.
For an oil film of 0.5 mm or 0.05 cm the extra volume needed will be 7.38 ml, for an oil film of 0.025 cm the extra volume needed will be 3.69 ml.
Now the reality is the various manufacturers ere on the side of caution and add more oil than may be necessary, but too much oil in the compressed air tank detracts from the internal volume available for air as it uses up internal capacity, hence in guns with a small tank capacity you need to not overdo the amount of oil added to the gun or you will increase the gun's compression ratio, oil like water being incompressible.
The manufacturer of a gun determines the amount of oil that is free to move around inside the gun after all the internal surfaces are fully coated, hence longer guns require more oil to be added and the values required are shown in tables. If a pneumatic gun remains stationary in one position for any length of time then the oil inside the gun drains to the lowest point in the gun leaving only the thinnest of films coating all the other surfaces, the thickness of this residual film being determined by the viscosity of the oil, the ambient temperature and any surface tension effects on the oil wetted parts. What we are interested in is the gun under usage conditions in which case the oil is being spread around inside the gun and coating of the walls is likely to be more uniform after the rest of the oil pools in the lower sections of the gun. That oil coating, like a film of paint, does nothing for gun lubrication during actual gun operation where we want oil to move to the sliding and load bearing surfaces, that oil coming from the "pooled oil" which is free to slosh around the gun's interior.
Any extra length in a (rear handle) pneumatic gun involves a longer inner barrel tube and a longer outer tank tube, so the incremental amount of oil required will be wetting that additional surface area on the tank and inner barrel walls. This additional surface area "Delta S" is easily calculated as (Dit + Dib + Dob) x pi x "Delta L"; where Dit is the internal diameter of the tank tube, Dib is the internal diameter of the inner barrel tube and Dob is the outer diameter of the inner barrel tube. "Delta L" is the additional length added to the gun and in the examples being looked at here I will assume that "Delta L" is equal to 10 cm.
Now for a Mares "Sten" Dit = 3.8 cm, Dib = 1.3 cm and Dob = 1.8 cm
Hence for a 10 cm increase in gun length "Delta S" = (3.8 + 1.3 + 1.8) x pi x 10 = 216.77 sq cm.
If we say that an oil film of 0.5 mm or 0.05 cm covers this extra surface area then the volume is 216.77 x 0.05 = 10.84 cubic centimetre (cc) or 10.84 ml. Thus for an oil film of half that amount or 0.025 cm the extra volume required will be 5.42 ml. Note that we are neglecting the annular shape of the oil film itself in calculating the additional oil volume, but the result will be close enough for our purposes.
Another example is the diminutive "Taimen" where Dit = 2.6 cm, Dib = 1.0 cm and Dob = 1.1 cm
Hence for a 10 cm increase in gun length "Delta S" = (2.6 + 1.0 + 1.1) x pi x 10 = 147.65 sq cm.
For an oil film of 0.5 mm or 0.05 cm the extra volume needed will be 7.38 ml, for an oil film of 0.025 cm the extra volume needed will be 3.69 ml.
Now the reality is the various manufacturers ere on the side of caution and add more oil than may be necessary, but too much oil in the compressed air tank detracts from the internal volume available for air as it uses up internal capacity, hence in guns with a small tank capacity you need to not overdo the amount of oil added to the gun or you will increase the gun's compression ratio, oil like water being incompressible.
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