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How much extra Oil?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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popgun pete

Well-Known Member
Jul 30, 2008
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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.
 
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We do not know what the oil film thickness will be that coats the walls of the inner barrel (both inside and outside) and the outer tank (inside only), but we can maybe figure it out from the published oil volume tables. The most recent table that I know of is that for the Salvimar "Vintair" models which should be identical to the "Sten" series in the alloy tubing sizes used if the Scubapro "Magnum" of which they are a clone is any guide.

Now the increment in oil quantity is 5 ml for the gun sizes going from 65 cm to 75 cm and 75 cm to 85 cm., hence using the "Sten" derived number for "Delta S" of 216.77 sq. cm. we can deduce that the oil film on the tubing walls will be 0.023 cm. Now of course it is not going to be as precise as all that and any small oil volume variation will not be critical, but this calculation gives us a "ball park" figure. As a rear handle pneumatic gun becomes longer any excess oil will occupy a smaller proportion of the gun's internal volume, so there will be no real penalty for the gun's compression ratio if the oil quantity is slightly more than necessary in those longer guns.

Another factor to consider is the time it takes "pooled oil" to run down along the gun's inner walls when the gun is turned vertically to muzzle load it. On a very long gun the "pooled oil" (or "sloshing oil") may take slightly longer to run down to the release mechanism where it will be needed, so the oil quantity is boosted. My guess is that is the reason for 10 ml being the increment for 85 cm to 95 cm on the Salvimar oil volume table, resulting in an oil volume of 45 ml. That is rather interesting as the longest Mares "Sten" models (over 110 cm in length) used to be quoted as needing only 35 ml of oil!

One thing to remember with the Salvimar "Vintair" is the upstream power regulator valve imprisons oil in the tank unless you open it from time to time by pushing the power regulator knob forwards in the gate, whereas with a "Sten" you usually leave the regulator knob in the rear position which allows oil to circulate throughout the entire gun. The equivalent situation in the Salvimar "Vintair" is the knob should be forwards in the gate, as can be seen in the Salvimar gun photo.
vintair models.jpg
 
I might as well add the "Predathor" oil quantities table here so that we can see that there is 10 ml of extra oil for each additional 15 cm of gun length.
Predathor diag..jpg
 
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