Now More Than Ever: The Importance of Machine Base Flatness
Now more than ever I’m convinced that many machines are installed on bases that are not flat – especially large, newer and more flexible machines on old bases.
Recently, while doing some training for one of our customers (KSB Pumps Inc.), we had an opportunity to do a little experiment with a base and motor. KSB Pumps Inc. is a large well known pump manufacturer who provides the complete units of base, motor and pump. For many of their customers they do the initial installation work. They also do pump overhaul work which in many cases includes the re-installation. They use an Easy-Laser® E710 for shaft alignment and now they have added a D22 Swivel laser transmitter that will allow them to also measure the base for Flatness. Many customers request complete units with the coupling installed and aligned before they are shipped. This makes sense because if it is out of alignment it could be damaged during shipping. Many more customers are now also asking that the bases be measured for flatness as well as being leveled during installation. The base that we are going to measure for flatness is a two level steel bed (pictured below).
Base Specs: Length – 104 inches; Width – 38 inches; Height (UL) – 16 inches; Height (LL) – 10 inches; Weight – 8915 lbs; Motor RPM – 3600. Note: Base is sitting on adjustable chucks. It has been roughly leveled.
We start on the upper level by establishing a flat plane by adjusting the laser beam at three points to zero. This is a machined area and there is no surprise that when we measure the fourth point, it is also zero. This means we know that the upper level is flat.
PHOTO (left) – Zero laser beam to first position. PHOTO (middle, right) – Adjust laser beam to second and third positions to find reference.
Next we measure the lower level in relation to the flat upper plane we have established. For speed we will only take one measurement per mounting area. However, normally we would take four readings to map out the complete foot pad area.
The procedure is simple and we finish quickly finding that the base does have a very small amount of twist. We have measured over the actual bolt hole and we easily place the small amount of shim on the low points to make the base flat in relation to the upper level. We are now in a position to install the pump and motor. (Note: Their customer could have requested a fully machined base which would have been flat but they have chosen a welded plate construction and we all know that welded plate is not precise when it comes to flatness.)
PHOTO (below) – Before we do the installation, we conduct a little experiment. We know the base is flat so if we place three 10 thou shims under three feet we will be creating a twist in the motor because it is not on a flat surface. We are actually creating an offset between the bearing – something I call internal misalignment. Once the pump and motor have been installed, we have misalignment because we have used 10 thou shim on three pads only, giving it 5 thou offset at the centre (bearing housing).
PHOTO (above right) – The motor is now sitting down on the base with the bolts loose and we do have a small gap of less than 2 thou shown in the display unit. However, we cannot measure right across the foot pad. So we tighten the hold down bolts then do a traditional measurement for Softfoot loosening each bolt checking for a gap under the foot with a feeler gauge. There is no gap detected with feeler gauge so we use the laser system to measure and the laser gives us a result of 1.5 thou gap across the diagonal plane where the 10 thou was missing.
With all laser systems, the softfoot measurement result is based on shaft movement/deflection. Although we use it and recommend it as a guide, you may not see a true result because the shaft movement is transferred through the machines casing which is flexible then through the bearing which has play to the shaft. The feeler did not work because the sheer weight of the machine pulled it down onto the base. Yes, on smaller stiffer machines the foot may spring up. But will it measure all of the twist that is in the base or just a portion of it? I think the only way to know for sure is to measure the base with the machines removed using an Easy-Laser D22 Swivel laser.