Core Formulas

How to Calculate Cycle Time, Throughput, and Scrap Cost for Bearings and Gears

The core formulas every gear and bearing shop needs, worked end to end with real units and where each input comes from.

Five calculations carry most of the planning load in a gear and bearing shop: cutting cycle time, grinding throughput, heat treat scrap cost, assembly labor, and end-of-line test capacity. Each runs on inputs you already have on the router or the timecard, but the arithmetic hides traps. This guide walks the math step by step with real units so you can reproduce it by hand or check what the Gear Cutting Cycle Time and Bearing Grinding Throughput calculators return. Every number below comes from a defensible source: a verified rate, a metered volume, or a rejection tag, not a catalog optimism figure.

Start with gear cutting cycle time. The base is blanks divided by a verified cutting rate in gears per hour: 180 blanks at 24 gears/hr is 7.5 hr of pure spindle-in-cut time. You then inflate by a setup and inspection allowance, expressed as a fraction, to cover fixturing, hob dial-in, first-article, and tooth gauging. At 18 percent, 7.5 hr times 1.18 equals 8.85 hr. The verified rate is the load-bearing input; pull it from a timed trial on this exact module and material. A worn or resharpened hob cuts slower, so the rate drifts batch to batch. The Gear Cutting Cycle Time calculator applies this two-step structure directly.

Bearing grinding throughput is a compounding-yield calculation, not a single division. Gross parts equal parts per cycle times available cycles: 12 parts/cycle times 520 cycles is 6,240 parts. You then multiply by two derates that stack. At 88 percent cell uptime and 96 percent first-pass yield, accepted output is 6,240 times 0.88 times 0.96, which is 5,272 parts. The order of multiplication does not matter, but you must apply both factors, not average them. The overall derate here is 5,272 over 6,240, or 84.5 percent. Bearing Grinding Throughput separates the 749 parts lost to downtime from the 220 lost to rework so you know which leak to attack.

Heat treat distortion scrap cost is additive, not multiplicative, and that trips people up. The scrapped value is parts times accumulated cost per part: 42 parts at $185 each, which reflects material plus all machining done before the furnace, equals $7,770. You then add the fixed furnace batch cost, $2,400 for energy, atmosphere, and fixturing, and the containment overhead of $950 for sorting and rework labor. Total is $11,120. Divide by 42 and the true cost per rejected part is about $265, well above the $185 raw part value. Use the Heat Treat Distortion Scrap Cost tool, and remember gears carry most of their value before hardening, so late scrap is expensive.

Gearbox assembly labor mirrors the cutting-time structure but in technician hours. Base time is units divided by a verified assembly rate: 36 gearboxes at 2.4 units/hr is 15 hr of bench work. The alignment and check allowance, here 22 percent, folds in shaft alignment, bearing preload, backlash setting, torque verification, and a spin or noise check. Fifteen hours times 1.22 equals 18.3 hr. That is labor time, not machine time; with two technicians the elapsed span halves but the 18.3 hr of labor stays fixed. Gearbox Assembly Labor uses the released build quantity, not the nominal order size, so hours match what actually hits the bench.

End-of-line capacity and lubrication fill round out the set. Noise Test Capacity mirrors grinding throughput: units per cycle times cycles, derated by stand uptime and first-pass NVH yield. Four units per cycle times 150 cycles is 600 gross; at 90 percent uptime and 97 percent yield you accept 524. Lubrication Fill Cost is variable plus fixed: 420 liters at $14.50 times a 100 percent fill share is $6,090, plus $375 handling equals $6,465, an all-in $15.39 per liter. Notice the pattern. Capacity tools multiply two derates, cost tools add a fixed charge to a variable base, and time tools inflate a base by an allowance factor.

Units discipline prevents most errors. Keep rates as parts per hour and multiply, or as hours per part and add, but never mix them mid-calculation. Percentages must become fractions before they multiply: 88 percent is 0.88, not 88. Allowance factors are one plus the fraction, so an 18 percent allowance is a factor of 1.18, not 0.18. When a result looks off by an order of magnitude, you almost certainly left a percent as a whole number or forgot the plus-one. Every calculator here shows both the base figure and the adjusted figure so you can trace exactly which step moved the number.

Sourcing each input matters more than the formula. A verified cutting or assembly rate comes from a stopwatch on the current part, not a spec sheet. Accumulated part cost for scrap comes from your standard cost roll-up at the heat-treat operation, not raw stock price. Uptime comes from the machine monitor or downtime log, and first-pass yield comes from inspection records over a recent window, ideally spanning a full dress cycle so a fresh-wheel yield does not flatter the number. Feed those into Gear Tooth Inspection Workload, Bearing Preload Setup Time, and the others, and the math is trustworthy. Bad inputs produce precise, confident, wrong answers.

Published 2026-07-01.