A Calibrated Pull Test Tool

[rgsparber]

I will be tapping 1/4-20 holes into what I think is bronze. It is essential that I do not strip out the threads as I pull on the eye bolt secured into this hole. My goal is to perform a pull test with a known force and then inspect the threads for damage.

I don't have a scale able to measure 250 pounds, so I decided to make a set of sacrificial bolts that will break at a known force. Here is the story.

Click here.

Your comments and questions are welcome. All of us are smarter than any one of us.

Rick

[DIYer]

Thanks rgsparber! We've added your Calibrated Pull Test Tool to our Fastening category,
as well as to your builder page: rgsparber's Homemade Tools. Your receipt:

Calibrated Pull Test Tool by rgsparber

tags: fasteners, tester

[Frank S]

Rick, I can understand your concerns about pulling the threads out of the bronze frames. Your method of testing the strength of the bolts will tell you if they are strong enough but you haven't mentioned the one critical factor regarding the strength of the material they will be screwed into. Which in your case happens to be your primary concern, of not pulling the threads out of the frame when dealing with similar materials such as a bolt and a nut it is generally thought that a nut of 80% thickness relative to the diameter of the bolt the male threads will fail before the female threads. If a bolt is threaded into cast iron or brass the machinery's handbook recommends 1.5 times depth or length of thread per diameter. Aluminum and tough plastics at 2 times. But when it pertains to castings a lot depends on the quality of the pour, but 2x generally as a rule of thumb will suffice if in doubt err on more depth of thread engagement, if at all possible, possibly 2.5 times if feasible.
With bronze depending on the age, you could be looking at as low as 12 to 15 KSI up to 28KSI on the upper end of the scale.
Would it be possible to drill and tap a hole somewhere that won't be seen or used to a depth that will allow thread length of 2x the diameter then screw a bolt in a torque to the specs of the bolt if the thread does not pull out at the max recommended torque, then you should be golden.

[rgsparber]

Rick, I can understand your concerns about pulling the threads out of the bronze frames. Your method of testing the strength of the bolts will tell you if they are strong enough but you haven't mentioned the one critical factor regarding the strength of the material they will be screwed into. Which in your case happens to be your primary concern, of not pulling the threads out of the frame when dealing with similar materials such as a bolt and a nut it is generally thought that a nut of 80% thickness relative to the diameter of the bolt the male threads will fail before the female threads. If a bolt is threaded into cast iron or brass the machinery's handbook recommends 1.5 times depth or length of thread per diameter. Aluminum and tough plastics at 2 times. But when it pertains to castings a lot depends on the quality of the pour, but 2x generally as a rule of thumb will suffice if in doubt err on more depth of thread engagement, if at all possible, possibly 2.5 times if feasible.
With bronze depending on the age, you could be looking at as low as 12 to 15 KSI up to 28KSI on the upper end of the scale.
Would it be possible to drill and tap a hole somewhere that won't be seen or used to a depth that will allow thread length of 2x the diameter then screw a bolt in a torque to the specs of the bolt if the thread does not pull out at the max recommended torque, then you should be golden.

Frank,

Thanks for your help.

I'm not sure we're on the same page here. My plan is to screw a calibrated bolt into the threaded hole in the bronze frame, which I will later use to lift the frame. Then do the pull test.

All other hardware associated with the lift is rated at 2 to 3 times the expected load.

By depth of thread, I assume you mean the number of threads cut into the bronze. This is part of my concern. That is why I want to test the individual threaded holes.

Where possible, I will be drilling a 1/2" hole and using SNAPTOGGLE anchors, which will spread the load on the back side of the bronze. In 1/2" thick steel, these anchors are rated at over 1200 pounds.

Isn't a torque test partially dependent on the friction between the nut and the threads? What is assumed? Such a test would let me avoid machining the necked-down section of these bolts, which is very attractive.

Thanks,

Rick

[Frank S]

Frank,

Thanks for your help.

I'm not sure we're on the same page here. My plan is to screw a calibrated bolt into the threaded hole in the bronze frame, which I will later use to lift the frame. Then do the pull test.

All other hardware associated with the lift is rated at 2 to 3 times the expected load.

By depth of thread, I assume you mean the number of threads cut into the bronze. This is part of my concern. That is why I want to test the individual threaded holes.

Where possible, I will be drilling a 1/2" hole and using SNAPTOGGLE anchors, which will spread the load on the back side of the bronze. In 1/2" thick steel, these anchors are rated at over 1200 pounds.

Isn't a torque test partially dependent on the friction between the nut and the threads? What is assumed? Such a test would let me avoid machining the necked-down section of these bolts, which is very attractive.

Thanks,

Rick

Perhaps I should have said depth of hole and length or number of threads.
A torque test would determine the shear strength or the point which threads in the bronze would strip out, but since you are planning on using the snaptoggle anchors I sense some confusion on my part. I was thinking you were going to drill #7 holes in the bronze and thread them to 1/4-20

[rgsparber]

Frank,

Sorry for the confusion. I will use a tapped 1/4-20 hole in the bronze where I must. But in some places, there is room to put in the snaptoggle anchor, which I prefer.

When I broke my calibration bolt with an estimated 275 pounds of force, it took very little torque. Do you have a reference for correlating pull to torque? From what I read, they aren't the same. I plan to finger-tighten my eye bolt into the threaded hole in the bronze, then pull rather than tighten it to the same force. My finger-tight eye bolt will have almost zero pull on the threads, while the lift will provide almost all of the pull.

Does this help clarify my situation?

Rick

[Floradawg]

Hi Rick, I don't know if you're already locked in to 1/4-20 due to existing parts. If not, 1/4-28 will be stronger. I'm sure you already know this.

[rgsparber]

Hi Rick, I don't know if you're already locked in to 1/4-20 due to existing parts. If not, 1/4-28 will be stronger. I'm sure you already know this.

Floradawg,

The forged eye bolts I'm using are 1/4-20, so I am locked into this size.

From what I've read, there is more thread depth with 1/4-20 than 1/4-28, so it is less likely for the threads to strip out. Conversely, the reduced thread depth of a 1/4-28 thread means the shank of the bolt has a larger diameter than a 1/4-20, so it is less likely to snap.

Is this consistent with what you have read?

Rick

[Frank S]

Torque testing as I mentioned would be to establish a clamping force between two members when trying to calculate the pull-out strength of the threads using torque on a bolt head as the method of determining thread pullout strength as much as 40 to 50% of the torque is lost in friction between the threads. To find the true strength of the threads to support a hanging load you need to first determine the minimum yield strength of the material, the area of a single internal thread at its base by multiplying the diameter by pi and the width of the base of the thread. then multiply the total number of threads in engagement by the shear strength of the material

Here is an AI generated summery which may be helpful to you

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Maximum Pull-Out Strength of a 1/4" Bolt in Bronze
For a 1/4" diameter bolt embedded in bronze, the pull-out strength depends on the embedment length and the bond/shear strength of bronze.

Step 1 – Material bond factor for bronze
Bronze is a relatively soft metal, so its bond factor (material factor MF) is lower than steel. A typical bond factor for bronze is around 300–400 psi a2zcalculators.com. For conservative design, use 300 psi.

Step 2 – Formula for pull-out force
The general formula is a2zcalculators.com:

F
p
o
=
π
×
D
×
L
×
M
F
Where:

F
p
o
= pull-out force (lb-f)

D
= bolt diameter (inches)

L
= embedment length (inches)

M
F
= material bond factor (psi)

Step 3 – Example calculation
Let’s assume:

D
=
0.25
in

L
=
1.0
in (10 mm embedment)

M
F
=
300
psi
F
p
o
=
3.1416
×
0.25
×
1.0
×
300
F
p
o
=
235.62

lb-f
So, for a 1/4" bolt in bronze with 1" embedment, the pull-out strength is about 236 lb.

Step 4 – Effect of embedment length
Doubling the embedment length doubles the pull-out force. For example, with
L
=
2.0
in:

F
p
o
=
3.1416
×
0.25
×
2.0
×
300
=
471.24

lb-f
Step 5 – Safety considerations
Use a safety factor (e.g., 1.5–2.0) for critical applications.

Surface finish, corrosion, and bronze type (e.g., tin bronze vs. beryllium bronze) can affect bond strength.

For high-strength bronze alloys, MF can be higher (up to ~400 psi), increasing pull-out strength.

Summary:
For a 1/4" bolt in bronze with a 1" embedment, expect a pull-out strength of ~236 lb at 300 psi bond factor. Increase embedment length or use a higher bond factor for higher strength. Always apply a safety factor in real designs. a2zcalculators.com

[rgsparber]

Torque testing as I mentioned would be to establish a clamping force between two members when trying to calculate the pull-out strength of the threads using torque on a bolt head as the method of determining thread pullout strength as much as 40 to 50% of the torque is lost in friction between the threads. To find the true strength of the threads to support a hanging load you need to first determine the minimum yield strength of the material, the area of a single internal thread at its base by multiplying the diameter by pi and the width of the base of the thread. then multiply the total number of threads in engagement by the shear strength of the material

Here is an AI generated summery which may be helpful to you

Copilot Search Branding

Like

Dislike
Maximum Pull-Out Strength of a 1/4" Bolt in Bronze
For a 1/4" diameter bolt embedded in bronze, the pull-out strength depends on the embedment length and the bond/shear strength of bronze.

Step 1 – Material bond factor for bronze
Bronze is a relatively soft metal, so its bond factor (material factor MF) is lower than steel. A typical bond factor for bronze is around 300–400 psi a2zcalculators.com. For conservative design, use 300 psi.

Step 2 – Formula for pull-out force
The general formula is a2zcalculators.com:

F
p
o
=
π
×
D
×
L
×
M
F
Where:

F
p
o
= pull-out force (lb-f)

D
= bolt diameter (inches)

L
= embedment length (inches)

M
F
= material bond factor (psi)

Step 3 – Example calculation
Let’s assume:

D
=
0.25
in

L
=
1.0
in (10 mm embedment)

M
F
=
300
psi
F
p
o
=
3.1416
×
0.25
×
1.0
×
300
F
p
o
=
235.62

lb-f
So, for a 1/4" bolt in bronze with 1" embedment, the pull-out strength is about 236 lb.

Step 4 – Effect of embedment length
Doubling the embedment length doubles the pull-out force. For example, with
L
=
2.0
in:

F
p
o
=
3.1416
×
0.25
×
2.0
×
300
=
471.24

lb-f
Step 5 – Safety considerations
Use a safety factor (e.g., 1.5–2.0) for critical applications.

Surface finish, corrosion, and bronze type (e.g., tin bronze vs. beryllium bronze) can affect bond strength.

For high-strength bronze alloys, MF can be higher (up to ~400 psi), increasing pull-out strength.

Summary:
For a 1/4" bolt in bronze with a 1" embedment, expect a pull-out strength of ~236 lb at 300 psi bond factor. Increase embedment length or use a higher bond factor for higher strength. Always apply a safety factor in real designs. a2zcalculators.com

1” embedment? Wow, that is massively thick.

I just ran my pull-out test. My necked-down bolt was set to break at about 500 pounds. I screwed it into the bronze which is about 1/4” thick and tightened the nut until the bolt snapped. Inspecting the threaded hole in the bronze showed no distortion and a fresh bolt smoothly screwed in with no noticeable side play.

Given an expected load of 150 pound, I believe I have a reasonable safety margin. AI’s calculation said it should have torn out.

Rick