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Torque Motor Calibration
by R.B. “Chip” Leadbetter, III, P.E.
Although the helical pile industry is not exactly new, calibration of installation motors has only recently become more than a wish and a dream. In fact, more than one person has asked me what calibrating a torque motor means and even more have asked me what benefits it can afford. The reason torque motor calibration hasn’t already become standard practice has to do with a variety of reasons including the technological abilities of calibration and the common installation equipment employed by installers.
As often occurs with newer or evolving technologies, the roots are simple. Helical installation equipment is no different. Torque motors are purchased with a theoretical torque curve which rely on hydraulic gauges on the rig to estimate the torque being applied. Early on, this was accepted as being as good as it could get. Minor improvements have been applied over time, such as employing back pressure gauges, but overall, the methods are basically unchanged. When the motor is new and the installer’s rig is new, the manufacturer’s torque curve is probably pretty close. However, over time the torque motors become worn, the pressure pumps in the rig become worn, pressure gauges lose calibration, and suddenly the torque may not be what you thought it should have been.
So the estimated torque isn’t quite right, what does it matter? In some situations, it probably doesn’t matter that much. However, as the industry becomes more seasoned, engineers increase their comfort level and leave less and less “cushion” in their designs. If the foundation location needs a certain load capacity, the installation torque needs to be sufficiently accurate to verify the required capacity exists. An older, un-calibrated torque motor may indicate adequate capacity was achieved, but because it is under-estimating, that helical should have been installed deeper and there is risk of the foundation settling. That is not something structure owners want to deal with.
The alternative could be to simply “over” install the helical - right? That might work sometimes, but since engineers typically select helical pile products right at the upper end of their capacities, installing to a greater torque than specified leaves the installer working in the range where breakage of the helical pile is possible. Breaking helical piles and anchors during installation is certainly not unfamiliar to contractors, but it does slow things down and adds cost to the project. Not to mention that a break near the surface is relatively easy to deal with, but one that happens at depth can become a real bear to resolve.
Having had the opportunity to actually compare the estimated torque with the actual measured torque on a variety of different contractor’s equipment, we have found that the manufacturer’s curves typically overestimate the torque being applied. The difference between estimated and actual measurements can be as much as 20% or more. That is right, 20%. If that installer was targeting 10,000 ft-lbs, he was actually only putting 8,000 ft-lbs into action. Everyone thought the foundations were just fine, when, in fact, every single helical on that job was not installed deep enough.
So instead of simply trusting installation motors and gauges, why not just put a measuring device into the system to eliminate the problem? Concepts of measuring torsional load have been available for a long time, but only recently has technology caught up enough to make it practical. Most load measuring devices (load cells) are connected to the power supply and a readout device by means of wire. For most situations, the wire is of no consequence. However, when applying this arrangement to a torsional movement, wires very quickly become wrapped up and expensive equipment becomes damaged. Recent advances in radio telemetry now enable the manufacturing of torque measuring devices that are wireless, and thus the device can spin indefinitely without the rodeo of wires.
Calibrating a torque motor is a relatively simple process. A wireless, in-line torque device is installed between the torque motor and the drive head, then a helical is installed in the ground. Measurements from the in-line torque device are recorded along with corresponding measurements from the hydraulic pressure gauges in the installer’s rig. With this data, a calibration curve can be created to indicate the actual torque being applied based on the pressure gauges.
Although we commonly refer to it as calibrating the torque motor, in reality the calibration is for the whole system. It includes the torque motor, the installer’s rig, the hydraulic gauges in the rig and everything that connects it all together. Change any one piece, such as changing out a broken gauge or replacing the hydraulic pump, and the calibration is no longer any good as the values on the pressure gauges are now different. Move the torque motor to a different rig, and the calibration again is of no value. In addition, time takes its toll on moving parts, pumps and motors wear out, so depending upon hours of use, calibrations are typically accurate for only one year.
There is plenty to consider when getting your equipment calibrated. Of course you want to make sure to calibrate every combination of torque motor and installation rig that you typically use. If the torque motor never leaves a particular piece of equipment, managing calibration becomes much easier. However, if you are using the same torque motor on several rigs, each combination must be calibrated. In addition, for a full calibration, torque measurements must be taken across the full range of potential torque loads. This means you need a test site where the soils permit you to achieve full torque on a helical pile or anchor. In all, it is a bit more than just showing up for roll call, and putting in the time to properly plan it out will help to get the most out of the calibration investment. Don’t hesitate to include your calibration agency in this planning effort, as they will often have good suggestions on how to make things simpler and avoid potential problems.
Where is the industry headed with regard to calibration? Some amazing strides have been realized in the last couple years. Where expensive load measuring devices used to be extremely rare, in-line torque devices are now available for purchase by installers themselves. While these devices are not exactly cheap, they are available and the interfaces are getting better and better. Theoretically, an installer could keep an in-line device on his rig at all times to measure and even record installation torque and never even use his pressure gauges except to monitor the health of the equipment. Expect this to become the standard on most helical pile and anchor installations in the coming years. In fact, expect to see these devices incorporated directly into the torque motors or in some way into the connection between the torque motor and the installer’s rig.
Keep in mind that even as these in-line devices become more common, they also need regular calibration. Simply investing in an in-line torque device does not eliminate the need for calibration. The in-line device simply gives more refined readings. You still need to make sure the number it spits out is actually what your motor is twisting. Since accurate, high torque calibration equipment is not very common across the US, having access to a small, separate load device has its benefits, as shipping an entire torque motor can be expensive. However, onsite calibration can also be used for in-line devices.
Torque motor calibration expectations are not going away. In fact, expect that calibration requirements to become a typical part of the helical pile construction specification. More and more municipal owners will be requiring calibrations, and as always happens, more and more private owners will follow suit. Calibrations are for more than just appeasing that “difficult to deal with” oversight engineer who keeps harassing you. They also provide the operator with confidence knowing the work he is performing is accurate and as specified. After all, that is what you are getting paid for - right?
About the author:
Chip Leadbetter is the Director of the Helical Pile Specialty Testing Group at CTL|Thompson, Inc., and the Division Manager for the Northern Colorado Branch. With a strong background in geotechnical engineering and a staff of highly capable structural, material and geotechnical engineers, his team provides engineering support and product testing to many helical pile manufacturers, distributors and installers.