Strain gauge alignment allows for shaftline measurement while afloat

Apr 11, 2016 by Rich Merhige

Aligning shafts using optical and laser techniques is pretty common when it comes to yacht maintenance.  There’s another form of alignment out there that gets a lot less attention, but has plenty of merits – strain gauge alignment. Strain gauge alignment is widely accepted technique of determining shaft alignment and is the only method for measuring the entire vessel’s shaftline with the vessel afloat.

The strain gauge technique provides shaft alignment information through the fundamental theory of flexural beam analysis. The shaft is considered as a beam, and by calculating the bending moments from the measured strain, the bearing reactions can be determined. There are two proven ways to calculate the bearing reactions (loads) using strain gauges, the free-body method and the moment-theorem method. The free body method divides the shaft into separate “artificial“ free bodies at each strain gauge measurement location.

Shaft alignment through the use of strain gauges measures the vertical and horizontal forces on the bearings in a shafting system. Strain gauges are placed on the shaft, which is then rotated in order to measure the load on the bearings. The measurements from the gauges give the bending strain of the shaft at a given location. Formulas for force and moment of equilibrium are utilized to relate the bearing reactions to the bending strain, whereas the bearing offsets for proper alignment can be calculated. The recorded data is then used to create a shaft alignment model which provides for precise calculations to achieve alignment.

Seems simple enough, but if it was, a lot more companies would offer this highly technical and precise service. There are several factors that contribute to successful testing, which are usually only appreciated by seasoned analysts. These parameters include: quality of the strain gauge instrumentation; quality of the strain gauge; correct selection of the strain gauge; bonding adhesive; environmental factors; installation of the strain gauges; circuit design; and the use of any other strain gauge accessories.  

The complexities of strain gauge testing begin with selecting the correct strain gauge. Selecting the proper gauge with the appropriate characteristics for the task at hand is extremely important to optimize strain gauge performance, obtaining accurate and reliable measurements, assisting in the installation process, and keeping the testing procedure cost effective. The selection process dictates that the strain gauge selected be most compatible with the environmental and operating constraints, while at the same time, being a viable option given and installation and operating constraints.

Strain gauges are usually defined by a set of parameters, including: Strain sensitive alloy, baking material, gauge length, gauge pattern, self-temperature compensation and grid resistance.

Environmental and operational parameters that need to be taken into consideration during the selection process include: Accuracy, stability, temperature, elongation, test duration, cyclic endurance, ease of installation and environment.

Once the strain gauge is chosen, then comes the installation process. Because the strain gauges are so sensitive, great care must be taken when installing them to avoid creep and damage to the gauges. Due to the sensitivity of the gauges, their performance is largely based the bond made between the gauge and the test subject during the installation process. Some methods of installation include using a glue, or adhesive, and welding or soldering them on.

Although the process of strain gauge alignment is highly precise with much needed attention to detail, there are many benefits to employing this method:

  1. There is usually no need to dismantle equipment.
  2. The process provides valuable data in terms of vertical and transverse bearing loads to stress, which is collected from multiple points, including the gears. This data is often impossible to collect through other methods due to inaccessibility.
  3. Once the installation process is mastered, readings can be obtained in about an hour.
  4. Measurements can be taken at periodically during the vessel’s life to insure safe bearing and gear alignment is maintained.

Strain gauge alignment is definitely not common place. However, with an experienced engineer or well trained analyst, this form of alignment can offer significant savings in time and money by eliminating time used to disassemble equipment, or the costs to haul a vessel since it could be done in the water. Being able to achieve alignment across multiple planes simultaneously also gives this method an advantage over traditional ones.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale. Contact him through