6" Dia Threaded Screw Jack
6" Dia Threaded Screw Jack
(OP)
Hi,
I am in the process of designing a heavy duty screw jack at the bases of a 6m tall support frame. We are designing it for approximately 400kN compression.
Conceptually the idea was to allow the torque / compression in the screw jack to be checked and adjusted over time from grade. The screwjack collar would sit on a seat (that keeps everything centered) with a Teflon coated surface to reduce friction.
Conceptually we were considering either a 4" dia or 6" dia threaded rod with a machined collar fabricated into the base of each 300mm dia. Pipe. This we were thinking would be turned using four - 2m long 1 1/2" dia rods or equivalent HSS's etc.
Looking at thread options for a 6" dia threaded round stock it looked like 6-16UN, Class 2A was the most applicable thread.
Normally to convert required tension to torque I would use the simple equation noted on the Portland bolts website among others however, for large diameters different thread pitches I imagine this would not be applicable.
We found this reference that had a more detailed equation that included TPI etc.
https://www.pdhonline.com/courses/s149/s149content...
However, when attempting to convert a determine the torque required to achieve a certain vertical force it appears it may become problematic to turn (torques potentially too large!).
Not sure if anyone has used extremely large diameter nuts / screws before and what the best way to tighten them in a controlled manner would be... Thanks!
Regards,
Reuben
I am in the process of designing a heavy duty screw jack at the bases of a 6m tall support frame. We are designing it for approximately 400kN compression.
Conceptually the idea was to allow the torque / compression in the screw jack to be checked and adjusted over time from grade. The screwjack collar would sit on a seat (that keeps everything centered) with a Teflon coated surface to reduce friction.
Conceptually we were considering either a 4" dia or 6" dia threaded rod with a machined collar fabricated into the base of each 300mm dia. Pipe. This we were thinking would be turned using four - 2m long 1 1/2" dia rods or equivalent HSS's etc.
Looking at thread options for a 6" dia threaded round stock it looked like 6-16UN, Class 2A was the most applicable thread.
Normally to convert required tension to torque I would use the simple equation noted on the Portland bolts website among others however, for large diameters different thread pitches I imagine this would not be applicable.
We found this reference that had a more detailed equation that included TPI etc.
https://www.pdhonline.com/courses/s149/s149content...
However, when attempting to convert a determine the torque required to achieve a certain vertical force it appears it may become problematic to turn (torques potentially too large!).
Not sure if anyone has used extremely large diameter nuts / screws before and what the best way to tighten them in a controlled manner would be... Thanks!
Regards,
Reuben
RE: 6" Dia Threaded Screw Jack
RE: 6" Dia Threaded Screw Jack
Thanks for the comment. What are your thoughts on balancing the load though during a jacking motion? Adjusting them evenly at the same time? Just coordinated 3/4 people coordinated turning nuts at the same time / risk of overloading one more than the other etc.... ?
Regards,
Reuben
RE: 6" Dia Threaded Screw Jack
RE: 6" Dia Threaded Screw Jack
RE: 6" Dia Threaded Screw Jack
Sorry if I'm being a pest about this, but it seems like the pancake jack idea would work well with the multiple threaded bars, though - 3 bars at the perimeter of a bearing plate with the pancake jack centered under the bearing plate.
RE: 6" Dia Threaded Screw Jack
Threads like 6-16UN are a very poor choice for screw jack type loading... way too much friction, as your calculations indicate.
Screw jacks use more efficient threads, square thread is normally first choice with trapezoidal thread (like Acme) being second choice.
Check with a mechanical engineer experienced in this type design to keep from coming up with a jack that won't work with the anticipated loading.
www.SlideRuleEra.net
RE: 6" Dia Threaded Screw Jack
RE: 6" Dia Threaded Screw Jack
RE: 6" Dia Threaded Screw Jack
400kN in compression is about 90 kips, is that per frame leg or for the entire frame? You really need to provide some sketches of the entire load and support frame, and of your concept of the lifting and leveling scheme you are imagining, because we can’t see it from here. Include plans, side views, dimensions, loads, member sizes, etc. etc.; to some reasonable proportions so as not deceive as to the overall arrangement; include a good sketch of how your 4” or 6” threaded rod mates with a collar and a 300mm pipe. Are you trying to lift or level the 6m tall support frame, or are you trying to determine its total weight, since you keep talking about a force/torque relationship, over time? Why does this need to be adjusted over time? If I were designing a screw jack, I would be using acme threads or some thread form near an acme thread form. And also, screw torque is a really poor way to measure the actual force being applied by the thread or jack. It is just not very reliable because of the system variables which you can not determine/control accurately in this type of application, and over time. You would design the jack to be strong enough to lift the desired load (plus a FoS) and then use lift distance as your measurement means, since you really already know the load you are lifting. In terms of coordinating a number of guys at the different legs, I would use a turn-of-the-nut method, where each guy applies one half turn or .75 turn at a time and then stops.
Your idea of a 50t hydraulic pancake jack directly under a hard/fixed lifting point on each leg (but not under a/the threaded rod) sounds like it has potential. 5 or 10mm of lift may not be too much temporary out of level or out of plumb on a sufficiently large (or flexible) frame. You lift it, uniformly +/-, to where you want it and put added shims under its true bearing points, +/- a couple mm. You might need some lateral guides at each leg base.
RE: 6" Dia Threaded Screw Jack
The frost layer comes into play for one foundation we can't get down below the frost level with that foundation due to how tight it is with existing equipment.
The idea was to allow the compression load to be monitored and adjusted every few months to mitigate this risk.
The arrangement has full lateral restraint for each leg. and lateral restraints at the load application area.
Yes agreed regarding accuracy and the pancake jack. I think lifting under the threaded rod should be fine though (refer to attached sketches) for reference.
https://files.engineering.com/getfile.aspx?folder=...
RE: 6" Dia Threaded Screw Jack