1.1 Characteristics of vertical hard-bearing balancing machines are described which make such machines suitable for gas turbine rotor balancing.1.2 This document specifies the following: a. general dimensions and capacities; b. performance requirements; c. balancing speed ranges; d. drive power requirements; e. balancing machine spindle flange dimensions; and f. test procedures.1.3 Proving rotors and associated test masses required for the performance tests are described in ARP4162.1.4 Test procedures are described in detail and test log samples along with a polar diagram for evaluating the test results are furnished.1.5 The predecessor document ARP588, ISO 2953, and the available technology of hard-bearing balancing machines have been considered in the preparation of this document.1.6 This document was developed for hard-bearing balancing machines but may also be used for existing soft-bearing balancing machines.