Neck torque

[David Houchens]

Wow, my brain is tired.

[kkmm]

Yep, on the practical sides, "use what has worked" and leave the calculations stuff to scientists, people who want to know why things work a certain way. I was a mathematician, but I love practical things so I chose Engineering as my career (and happily retired recently).

[Larry Simonson]

Many years ago someone told me that it is the little button's attachment at the neck heel that kept the neck on a violin, so I have been interested in how that applies to mandolin. With this in mind I have done an experiment of simulating a mandolin neck heel button joint, then seeing how much torque it can withstand. In the pictures below I used a fulcrum of 1.56 inches which is the height of the neck block (1.375") plus plate thickness (about 0.2"). There can be different opinions on this choice ranging from just the neck block 1.375" to the top of the neck of about 2" , depending on how much one thinks the under fretboard extension is supporting. My choice of 1.56 seemed reasonable. The button tab was glued on with tight bond glue. The size of this glue patch was 1.5" by 0.6" with one end rounded off. These dimensions were taken from Simonoff's original book. I allowed to simulated neck to to extend well past the 8 inches of neck to nut and hung a 5 gallon pail at 12 inches from the simulated joint. I then added water to the pail. Results: The joint held when the pail contained about 5 gallons of water. The pail and its contents weighted about 42 pounds. After 24 hours it was still intact and the experiment was terminated.

I don't know what all this means but it seems that that little button joint can resist 42 foot pounds of torque for some period undetermined. Since the torque arising from the string tension on a mandolin using J-74's is on the order of 10% of 182 lbs at 8 inches or about 12.2 foot pounds, it is tempting to conclude that the little heel button is sufficient to stabilize a mandolin's neck. Of course there are other glued surfaces in a real neck and there is the problem of glued joints creeping that need to be considered in the long term.

[kkmm]

I love this experiment, it does give the answer that one need with proper parameters set in the experiment.
This morning, I did an experiment to convince me that the "under the saddle" transducer works on a silent guitar (which I am building from scratch). It does not need a resonating body as the string vibrations are picked by the transducer and thru a preamp then an amp, the sound is produced. This is the same transducer seen on acoustic / electric guitar which has a resonating body.
The silent guitar does not, and the experiment allows me to proceed with my little project (I don't have all the parts yet, they are being shipped).

[Bernie Daniel]

Many years ago someone told me that it is the little button's attachment at the neck heel that kept the neck on a violin, so I have been interested in how that applies to mandolin. With this in mind I have done an experiment of simulating a mandolin neck heel button joint, then seeing how much torque it can withstand. In the pictures below I used a fulcrum of 1.56 inches which is the height of the neck block (1.375") plus plate thickness (about 0.2"). There can be different opinions on this choice ranging from just the neck block 1.375" to the top of the neck of about 2" , depending on how much one thinks the under fretboard extension is supporting. My choice of 1.56 seemed reasonable. The button tab was glued on with tight bond glue. The size of this glue patch was 1.5" by 0.6" with one end rounded off. These dimensions were taken from Simonoff's original book. I allowed to simulated neck to to extend well past the 8 inches of neck to nut and hung a 5 gallon pail at 12 inches from the simulated joint. I then added water to the pail. Results: The joint held when the pail contained about 5 gallons of water. The pail and its contents weighted about 42 pounds. After 24 hours it was still intact and the experiment was terminated.

I don't know what all this means but it seems that that little button joint can resist 42 foot pounds of torque for some period undetermined. Since the torque arising from the string tension on a mandolin using J-74's is on the order of 10% of 182 lbs at 8 inches or about 12.2 foot pounds, it is tempting to conclude that the little heel button is sufficient to stabilize a mandolin's neck. Of course there are other glued surfaces in a real neck and there is the problem of glued joints creeping that need to be considered in the long term.

Great experiment!

[sunburst]

The experiment (and a strung-up mandolin) subject the glue joint to sheer force. Here's another plug for hot hide glue; hot hide glue resists sheer force (the force that causes creeping) better than just about any other common glue or adhesive. To further the experiment, one could glue up similar pieces with various different glues, then leave them for months or years. During those months and years, subject them to the extremes of heat and cold that a mandolin is likely to encounter, and maybe even subject them to a few bumps and shocks like a mandolin is likely to encounter in use.
It is known that a good joint between the neck heel and the back plate strengthens the neck joint, especially in violin where there is usually no mechanical joint (like a dovetail), just the glued mortise and tenon, but there are plenty of examples of neck joint holding well without that feature. Most flat top guitars, some mandolins, others.

[Larry Simonson]

Thanks John, I had the same thoughts about shear force and creeping. You are right, its another reason to use HHG.