Homemade Spring Tester

[JohnKSa]

Specifications
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.



Description
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.

http://www.gunsprings.com/index.cfm?...s&cID=1&mID=52

Seems to work ok.

[Double Naught Spy]

Very cool! So what guns use 110 lb recoil springs?

So you will use this to measure recoil spring use-life?

[JohnKSa]

I actually already had the scale to use as a luggage scale and it has made a couple of flights around the country making sure my wife's luggage doesn't get too expensive at the airport. I've saved enough in overweight bag fees to pay for the scale several times over.

However, when I initially acquired it, I was also thinking that I might be able to find a way to put it to other uses. So I wanted the high weight capacity for luggage but the accuracy for uses like this.

When I first started mulling over the idea of building a spring tester some time back, it was because I wanted to be able to run a test on magazine springs, and I still plan to get around to that. But in the interim, I've also gotten more interested in the dynamics of recoil-operated autopistols so I'll probably use it to gather some data to help me better understand that topic.

[dahermit]

Someone please send a spring tester to Colt Q.C., quick!

[g.willikers]

Very nice.
I use one made with a small fish weighing scale.
The problem with a scale of too larger a range is that most scales are the most accurate at about the center of the range.
Trying to get an accurate reading for most gun springs, from a 110 pound scale might not be meaningful.

[wachtelhund1]

JohnKSa, Great post, I'm going to one to test some springs real soon.

[JohnKSa]

... most scales are the most accurate at about the center of the range.

I can certainly see how that would apply to traditional scales that compress an internal spring and measure deflection based on the compression distance, but I'm not sure that applies to scales that use electronic load cells.

But, why speculate when one can experiment!

So I got the scale out and dug up a set of calibration test weights. Below are the results of weighing a 100gram test weight.

In "oz" mode the readout provides 1 digit after the decimal point and the scale reading was: 3.5oz (3.5oz=99.22grams. 100 grams is about 3.52736 oz. The scale is accurate to the last digit on the readout.)

In "lbs" mode the readout provides 2 digits after the decimal point and the scale reading was 0.22lbs: (0.22lbs=99.79grams. 100 grams is about 0.22046lbs. The scale is accurate to the last digit on the readout. HOWEVER, the scale's manufacturer only claims it is accurate to 0.02lbs in "lbs" mode. It was just luck that the number came up to be an even hundredth of a pound. For very precise measurements, one should put the scale in "oz" mode.)

So the manufacturer's stated accuracy/resolution figures quoted in the initial post appear to be correct, even at the extreme low end of the weight's operational range.

I've added a caution note to the original post for those who decide to use a different scale in their tester.

[JohnKSa]

Ok, I've revisited my initial design after using the tester for awhile.

The initial design can be improved in the following ways:

1. Drill a hole in a wood block and clamp the wood block in a vise. The wood block becomes the reference point and replaces one set of washers and the brass collar. The spring guide goes through the hole in the block, then through the spring and then through the compression washer. It makes things a little easier to deal with.

2. Add a small nut on the spring end of the compression washer(s) to stabilize the compression washer(s). Otherwise the compression washer(s) will tend to tilt to one side or the other resulting in inconsistent measurements.

Those two changes, especially the last one will improve the design enough to make it very workable for someone who wants to do a spring measurement once in awhile.

BUT, I decided to run another magazine spring test after posting the results of the first one here. The new test will involve 4 magazines. The prospect of trying to use the original design to repeatedly measure the 4 springs was unappealing so I came up with a new design.

This one uses a rigid frame with a sliding block enclosed in aluminum rails. This design prevents the spring or the compression block from torqueing to one side and allows very accurate and repeatable compression distance. The result is more consistent/accurate results.

I'll try to get some pictures posted in the next few days. Unfortunately this one isn't as simple to put together as the old one was.