I took delivery of my Schenck HT0B balancer coupled with a CAB720 display direct from the Schenck factory in Germany. This balancer was originally used to balance high speed electric motor armatures and has been totally updated and calibrated before shipping to me. This model of balancer and display is used by Simjet in Denmark and recently one was acquired by Kerry Sterner in the USA who is also the Simjet distributor there to balance all of their turbines whether new or repaired. Simjet have always been renowned for having the best engine balance in the business . The secret to this is the HT0B and the CAB720 display. This option allows graphical representation of out of balance it also has special software to remove bearing noise and is super sensitive and highly accurate. The balancer was commissioned by the Schenck agent here in Australia in September 2007 where he also gave myself and Philip Collings training on the unit for a full day. We have used the balancer now and balanced around 15 turbines to get practice and have come to the conclusion that this machine is just spectacular and worth every cent I paid for it. Now to the nitty gritty of why I bought this particular balancer.
The main way of balancing a turbine is to remove the compressor off the shaft then remove the shaft and turbine wheel from the housing, replace the compressor on the shaft to comprise what is commonly called a “dumb bell” obviously because it looks like one. To balance this assembly with a normal balancer it will be placed on a set of fixed bearings in the balancer and then spun up with a belt system and you remove material from the front and the rear until the assembly is within whatever tolerance you have set. The main issue is that to put the assembly back inside the casing you have to remove the compressor then reassemble the engine. Once you do this the practicalities of getting the compressor back in the exact same position on the shaft as when it was balanced is almost impossible. This means that the engine is not balanced to the same tolerance as when it was done as a “dumb bell”, so the engine is not balanced as it should be.
The method of balancing we use with my Schenck is similar in how you actually get to the balance but it is different in the outcome. What we do is referred to as a “core” balance because we balance the completed core of the engine, this incorporates the bearings and casings used that the engine actually runs with. We don’t have to pull the engine apart to balance it. If you get a ding or nick out of your compressor the engine is severely out of balance and we can rebalance it without taking the engine apart or a worse case scenario is you have a crash and the engine is damaged or full of dirt. You can send it to us we will strip and clean it out replace any damaged parts then reassemble the engine, place it in the balancer and balance the whole engine assembled. So once the engine goes back together it stays that way. When we are finished the balancing the tolerance we achieve is the actual tolerance when the engine is running.
The engine is placed in a special holder in the balancer and the critical measurements of where we take off the material on the turbine wheel and compressor to balance the engine are input to the computer in the balancer based on reference points that are on the balancer base. Once this is done we spin the engine up with compressed air from the compressor side to around 4000 rpm, once this speed is achieved the balancer takes 2 seconds to take its measurement and you can see this on the display of the actual imbalance weight and at what angle it is from the 12 o’clock position. The 12 o’clock position is determined by a special reflective tape we put on the turbine wheel that is picked up by a infra red sensor and this gives the balancer a datum reference to work from.
This engine is my very first turbine and has not been used for some 4 years or so and been in a cupboard. It was balanced last by a homemade balancer and not by myself or Phillip. The picture below is what the imbalance is when first run up on the Schenck. The turbine wheel is out by 23.5grams/mm at 31 degrees and the compressor is out by 15 grams/mm at 27degrees.
This display above shows effectively the next two adjustments. The imbalance was bought from 19.1 down to 11.7 mg and then to 3.12mg but we can go back and see where the previous adjustment was compared to the current one. You can see on the vector display that we are on the right track of getting towards the centre of the vector
The next series of pictures show what happens as you get closer to the tolerance I have set which is to balance all turbines to under 1 gram/mm
As you get very close to having one end balanced making small adjustments on the compressor can change the actual position and amount of imbalance at the other end. This where core balancing comes into its own. You can see if you got to a nice balance with a dumbell setup as soon as you remove the compressor off the shaft then reassemble the engine again if you don’t get the compressor in exactly the right place the assembly will be out of balance at either one end or the other. This of course is not taking into consideration that the bearings the dumbell will be put into are not the bearings the balance was done in. The last picture shows the final balance is within the tolerance I have set. The two green blocks in the centre of the display tell me it is under 1 gram/mm as this is the tolerance setting that we set in the balancer. We could balance to lower than this as the balancer is well capable to do so ( .02 grams/mm) but the amount of material we are taking off the metal is so small at this level it is almost impossible to physically do this
