June 12, 2017
Correcting for base bound or bolt bound conditions.
By Tom Shelton
A question often asked during our training classes is how to deal with base bound and/or bolt bound conditions when performing a precision shaft alignment on rotating machinery.
Base bound is the vertical condition that occurs when the moveable machine needs to be lowered but is “bound” by the base due to an insufficient number of shims (or no shims) under the movable machine’s feet. Depending upon the type of base the options are to; machine the foot pads on the base, replace the movable machine’s sub base, machine the feet of the movable machine, or move the stationary machine if possible.
Machining the base is not always practical due to the costs and time constraints, etc. Machining the motor feet is not recommended as now the motor is a “special” and non-machined spare motors no longer fit this application and the feet could possibly be machined too thin to properly support the motor.
Bolt bound is the horizontal condition that occurs when the moveable machine is “bound” by the hold down bolts in the direction of movement needed to correct the horizontal alignment. Typical remedies to correct bolt bound are to; drill out the movable machine feet, undercut the hold down bolts, replace the hold bolts with smaller diameter ones when through bolted, or move the stationary machine if possible.
Drilling out the holes on the motor feet can lead to other issues such as cupped washers, and the application now requires a “special” motor. Undercutting the hold down bolts by removing threads on the shank of the bolt offer a little movement side to side which many times is not enough to resolve being bound, using smaller bolts and nuts (when through bolted) can give enough side to side movement however too small of a diameter bolt may not have enough tensile and yield strength for the application.
Moving the stationary machine (if possible) can be a proper and quick solution to solving base and bolt bound conditions, however the question is always how much movement is needed? It’s not as much as one would think and typically requires only a slight rotation in the vertical or horizontal plane of the stationary machines.
Fixturlaser has made this a simple issue to analysis and correct in most circumstances. The Fixturlaser NXA uses a common-sense approach to solving bolt or base bound conditions with the Feet Lock function. The power of this function is that the mechanic can choose which two sets of feet to consider “stationary” and the NXA will calculate values needed to move the “unlocked feet. Most times there is a combination of locked and unlocked feet to solve the bound issue.
Some situations are absolute as there can be no adjustment made to the stationary machine as in the case of a reciprocating gas compressor however there are thousands of other machines which may be corrected using Feet Lock.
Once the correction process is begun the Feet Lock function is available by pressing the Configuration Icon then the Feet Lock icon.
After entering dimensions for the “stationary” machine the user will select which feet to lock on one of the following screens.
Base Bound Screen Bolt Bound Screen
As previous blog discusses using Feet Lock to correct for Base Bound (Link to Base Bound blog). This blog will demonstrate solving for bolt bound.
After the vertical correction was made, the live screen shows that the movable must move left as viewed from the non-drive end of the movable. The movable cannot be moved left as it is up against the hold down bolts. Now we can use the Feet Lock function to determine our options.
We will choose which sets of feet to keep stationary and which to move based on the machine and the ability to move it. Choose feet until you find an option that allows the motor to move right. Out of the bolt bound condition.
This option is not feasible as the movable still needs to move left.
This option doesn’t work either.
This one works! We only have to rotate the stationary 18 mils and the motor 15 mils to correct for the original 53 mils of restriction when the stationary feet were locked.
This is just one of the functions in the NXA designed to help the mechanic be more efficient and effective in fulfilling his obligation of reliable machine operation at the plant.
Save Time. Save Money. Save the Machine.
To find out more about the Fixturlaser NXA Pro or NXA Ultimate go to: https://acoem.us/tools/shaft-alignment/