How Good Is Your Balance?
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How Good Is Your Balance?
by Ronald L. Eshleman, PhD
Abstract
This paper examines the relationship between field vibration balance measurements to the balance grade designations described in ISO 1940-1. The author explains this relationship by use of an example: a single-plane balance of a FD fan. Both the balance sensitivity and phase lag are calculated. Final vibration measurements are used with balance sensitivity data to calculate the residual unbalance left in the fan wheel. This residual unbalance is compared to the ISO designations to determine what final balance grade was achieved.
PREVIEW
“The quality of most shop balances is judged by ISO 1940 [“Balance Quality Requirements of Rotating Rigid Bodies”: Part 1. “Determination and Verification of Balance Tolerance,” ISO 1940-1]. However, the quality of a balance in the field is typically judged by the measured vibration. It is worthwhile to relate the balance of a machine to ISO 1940 with a field balance. This process is illustrated with a balance conducted on a 1,780 RPM forced-draft fan weighing 1,550 pounds. The radius of the balance weight was 23.5 inches. The Figure shows the results of the balance. The initial vibration readings on the inboard and outboard bearings were 3.3 mils pk-to-pk at 32⁰ and 3.7 mils pk- to- pk at 35⁰ respectively, yielding an original vector O = 3.5 Mils at 34°. After the addition of a trial weight of Tw = 3.3 oz at 66⁰, the readings at the inboard and outboard bearings were 0.9 mil pk-to-pk at 10⁰ and 0.2 mil pk-to-pk at 310⁰ respectively. The response vector (T) was 3.2 mils pk-to-pk at 219°. The response vector was moved 5° CCW, and 3.6 oz (3.3 oz x 3.5 mils/3.2 mils) were used to balance the rotor. The balance sensitivity were 1.03 oz/mil (3.3 oz/3.2 mils) at a phase lag of 153⁰ (360° – 241° + 34°) of the high spot to the heavy spot. The vibration level is relatively low. But what is the quality of the balance? Because it is essentially a single-plane balance, the results can be related to ISO 1940.
The formula that relates balance grade (G), rotor weight in pounds, and speed in RPM to permissible residual unbalance (pru) in oz-in. is
pru = 6.015 G W/N
pru = 6.015 x G x 1,550 lb/1,780 RPM = 5.24 G oz-in.
The balance sensitivity, bs = (Tw x R)/T, can be used to calculate the ending residual unbalance (ru) using the measured static vibration level (v = 0.5 mil pk-pk; see the Figure).
ru = bs x v = Tw x R x v / T
ru = 3.3 oz x 23.5 in. x 0.5 mil/3.2 mils pk-pk
ru = 12.12 oz-in.
Therefore, the balance grade − an ISO indicator of balance quality − can be obtained by equating pru to ru (the residual unbalance): 5.24 G = 12.12 G = 2.31 The recommended shop balance for fans in ISO 1940 is G = 6.3. The fan was in better balance than the shop balance recommendation.”
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