TECHNICAL INFORMATION LETTER - SMGSPG (121398)
Tippmann SMG Series, Bolt Main Springs
REVISION C - DESTROY ALL OTHER COPIES
SOURCE OR CAUSE: Different steels, coil diameters, wire sizes, coil spacing, spring length, heat treatment and tensile strength,singly or in combination, will give different spring energies. The supplier's changing of a machine and/or machine operator between orders may also have an effect on each different lot of springs. A long spring may well be of a lower power than a short one.
In measuring the static energies of a particular set of springs in both standard and high pressure, we found that they varied from 2 1/2 to 3 pounds added energy in the standard, and 3 1/4 to 4 1/4 pounds in the high pressure. The averages were 2 3/4 pounds and 3 3/4 pounds. The maximum difference's between different lots of the same type were as much as thirty-five percent!
REPAIR OR PROBLEM REDUCTION: Simple measuring of the spring length or its superficial color will have no bearing on the spring energy as the factory can only specify an energy at a certain compression length to the supplier and take what they get. Springs normally cannot be identified by simple means.
The uncompressed length of the springs ranges from 180mm to 200mm. At the rest, or un-cocked bolt position, the spring is compressed to 137mm. In the armed, or cocked bolt position the spring is further compressed 50mm more to 87mm. The difference in measurement is not kinetic energy. When the bolt hits the gun valve body, it has gained a lot more force. It is however, sufficient for our purposes.
To measure yours, obtain a simple kitchen or mail scale which has a range of ten pounds or more. Slide the spring over a 1/8" rod of nine inches or more in length. If possible, thread a 6X32 nut on the end to keep the spring from forcing off the end and then bending upon release. Slide a 1/8" washer over the rod on top of the spring the a larger washer or spacer to gain grip. Place the nut end of the rod in the center of the scale top and with a metric ruler, compress the spring to 137mm. Read the weight from the scale. Continue compressing to 87mm and read the scale weight again. Subtract the 137mm reading from the 87mm reading. This is the static or added energy of the spring.
The above is all academic though. All guns are different. The prime rule in both internal and external ballistics is, "You can engineer the hell out of a weapons system, but you never know what it will do until after you pull the trigger." The moral is simple. Clean your gun, lube it, then take it out to your field's chronograph. That is the only way you will know what velocity that spring will give in that gun.
Before chronographing, wait until after the temperature of the gun, CO2 cylinder and paint have stabilized. Normalize your gas system with three shots pointed up in the air, and then shoot three round sequences holding the barrel level without pointing it down until done.Keep the muzzle well to the rear of the light cell to prevent falsing. Keep in mind that changes in the ambient temperature, and hence the CO2 pressure will vary velocities considerably. Guns equipped with an All Weather Tank will record higher velocities with the same spring. You should make a set of springs cut to different lengths.
To adjust spring energies, cut off several coils at a time and retest. Use nippers, not your side cutters if you value them. Do not attempt to increase the tension by stretching the spring. You will be torquing the wire coils beyond design specs and weakening them. The length will recede in time and the spring will have less energy than it started with. If you want more tension, order one or more high pressure springs and chronograph them to get the tension you desire. USE ONLY FACTORY SPRINGS! Spring design is very intricate. One that, "looks about right", may eventually damage the gun valve internals, receiver, or end cap threads. Cutting more than 10% of spring length may give erratic velocities. High pressure springs cannot be cut down far enough to equal a low pressure one.
To verify one claim that adding spring coils to a regular spring increased velocity, we conducted our own experiments. We found that very little extra spring can be added as there is only a small amount of available room. The additional spring coils crowding into this space resulted in a binding and over-compression of the springs, AND A NET LOSS OF ENERGY OF APPROXIMATELY 10%! Therefore, only a spring DESIGNED with a certain amount of energy, at a certain compression will work in the SMG Series of guns.
RELATED INFORMATION: All SMG-68's were equipped with the standard 2.5 lb. spring until late 1988. All are now equipped with the 4.0 lb. high pressure spring. Some guns sneaked thru with 5.0 lb. winter springs. These yield 400 fps in summer and are dangerous. SMG-68's and Special's normally only use a standard pressure spring. Springs must be kept very clean and well lubed, as they will wear from friction and lose energy over a period of time. See also TIL-GENVEL, TIL-SPCVEA, TIL-SMGCLN and TIL-SMGOIL.
This document is retyped from a low quality copy that was hard to read. I don't type but the information is so important that it had to be done. Note that because the 68s are liquid fed, seasonal spring changes are not needed. SML
Blogger.com user StrayBlackCatsMeow
Address http://68-special.blogspot.com/
Blog "The 68-Special from Tippmann Pneumatics Inc"
Blog Reference page number 04
Title - TIL-SMGSPG
Revised - 10/18/14
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