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Old May 25, 2013, 08:49 PM   #1
Join Date: February 12, 2001
Location: DFW Area
Posts: 21,790
Homemade Spring Tester

Capacity: Approximately 110 lbs
Accuracy/Resolution*: 0.1oz in "oz" mode, 0.02lbs in "lbs" mode.

Spring Type Accommodated
Compression Springs (Recoil springs, magazine springs, etc.)

Spring Size Accommodated
Minimum Inside Diameter: 0.265”
Maximum Outside Diameter 2.0” **
Maximum Length About 10” ***

*Note that g.willikers brings up a good point about scales in post #5 below. I have tested the specific scale used for this tester (after reading his post) and verified that it is accurate to the quoted specifications above, even with very light test weights. If you use a different type of scale to do your weight measurements, you should read his post and take heed.
**To test springs with larger outside diameters, larger adapters (washers) are required.
***If some precompression is acceptable for the specific test to be done then longer springs can be tested.

Here's a picture of the parts.

Parts cost about $40 with the Jennings V2-50 Ultrasport Luggage Scale ($27.90+ shipping) being the primary expense.

The main guide rod is a 0.2” outside diameter threaded rod with a loop bent into one end. The looped end was dipped in black plastic to cover the threads.

The rest of the rod, with the exception of about half an inch on the non-looped end is covered in 0.25” outside diameter, 0.17” inside diameter polyethylene tubing threaded onto it. The only thing that’s remotely tricky during assembly is threading the tubing onto the rod. If you can find an unthreaded rod with a threaded end, or if you can thread one end of an unthreaded rod, that would eliminate the tubing and would likely allow the testing of smaller inside diameter springs.

The brass object is a collar that goes on first and runs down to the loop end. Then one set of washers goes on, then the spring to be tested, then the next set of washers, and then it's capped with the wing nut which screws on the end that isn't covered with tubing. The collar is clamped in a bench vice, and the luggage scale hook goes through the loop.

Then you pull on the luggage scale until the spring is appropriately compressed and read the weight from the luggage scale. You can mark on the polyethlene tubing with a grease pencil or with masking tape to establish fixed compression distances to make it easier to get the compression length correct for your measurements.

If you wanted to get really fancy, you could mark the rod like a ruler to make it easy to pull exactly the right amount of compression distance when doing your measurements.

It's not that easy to hold everything in position so that the spring is compressed exactly the right amount while reading the luggage scale at the same time, but the luggage scale I chose has a "Hold" button. When everything is lined up right and the spring is properly compressed, you can hit the hold button, let everything relax and read the figure off the display. It would probably be wise to repeat the process a few times and average several resulting raw measurements to get your final answer.

Quick Sanity Check
I broke down my Ruger P89DC and took out the recoil spring. I checked the spring preload distance and measured the spring weight at that length using the spring tester.

Here's a picture of the spring tester with the Ruger P89 recoil spring installed.

The force measured with the spring compressed to the preload length was 4.5lbs which means that the recoil spring is exerting about 4.5lbs of force against the slide to keep the gun in battery.

Then I measured the slide travel of the Ruger P89 from battery to fully retracted--2". Measuring the spring force with the recoil spring compressed 2" beyond preload distance gave a value of 10.8lbs of force. That means that the Ruger P89 spring has a spring rate of a little over 3lbs per inch

(10.8lbs-4.5lbs)/2" = 3.15lbs/inch

According to Wolff, the Ruger P89 factory recoil spring is rated at 11lbs. My measured figure of 10.8lbs at full operational compression agrees well with that number.

Seems to work ok.
Attached Images
File Type: jpg SpringTesterPieces_Small.jpg (164.6 KB, 421 views)
File Type: jpg SpringTesterwSpring_small.jpg (139.6 KB, 410 views)
Do you know about the TEXAS State Rifle Association?

Last edited by JohnKSa; May 26, 2013 at 01:49 PM. Reason: Added note about scale accuracy.
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