Cost & Quoting

What Really Drives Cost Per Unit on Gears, Bearings, and Gearboxes

A money-first breakdown of what drives cost per unit on power transmission parts and how to quote it without leaving margin on the table.

Cost per unit on power transmission parts is dominated by machine time and late-stage scrap, not raw material. A carburizing-grade alloy steel blank might be $8 to $25, but the hobbing, grinding, heat treat, and inspection stacked on top routinely push a finished gear to $150 or more. That is why the accumulated value at heat treat, not the stock price, is the number that matters when a part distorts. A defensible quote starts by assigning cost to each operation in sequence and refusing to let any large driver hide inside a blanket overhead percentage.

Machine time is usually the single biggest line. Price it as verified cycle time times a fully burdened machine rate. Gear hobbing and gear grinding spindles commonly carry burdened rates of $75 to $180 per hour depending on age, tolerance class, and automation. If Gear Cutting Cycle Time returns 8.85 hr for a 180-piece run at, say, $110 per hour, that is $973 of cutting, or $5.40 per gear before grinding. Precision Grinding Cost prices the finish operation that brings the part to its AGMA class after heat treat, and on ground gears it often rivals or exceeds the cutting cost.

Scrap is the driver estimators chronically underprice. On power transmission parts, most scrap happens after the most expensive operations, so it costs far more than a scrap allowance based on part value suggests. Heat Treat Distortion Scrap Cost shows a 42-part event totaling $11,120, about $265 per rejected part against a $185 part value, once the fixed furnace batch and containment burden spread across the rejects. Fold a realistic distortion loss into the quote as a cost, not a footnote. A 3 percent scrap rate on a part that has $140 of value at heat treat adds roughly $4.20 per good unit, and that is before containment.

Labor on the assembly side is quotable but easy to shortchange. Gearbox Assembly Labor turns a 15 hr base into 18.3 hr once shaft alignment, preload, backlash, and functional testing are counted. At a loaded assembly labor rate of $45 to $65 per hour, that 3.3 hr allowance gap is $150 to $215 of real cost per 36-unit run that a base-time quote would miss. Bearing Preload Setup Time captures the analogous setup burden on the bearing side. Quote the allowance-inflated hours, never the bench-only base, or you staff the cell short and bleed the margin back in overtime.

Consumables and end-of-line testing are small per part but real, and they compound at volume. Lubrication Fill Cost shows synthetic gear oil and greases at $14 to $30 per liter or pound, and a 420-liter batch runs $6,465 loaded, about $15.39 per liter after handling. Over-fill quietly inflates this and also causes churning heat, so meter to spec. Noise Test Capacity and End-of-Line Test Takt determine how much test-cell time each unit consumes; if the NVH booth runs slower than assembly, its burdened hourly rate gets spread over fewer units and per-unit test cost climbs.

Overhead should be allocated by the driver, not smeared as a flat percentage. Grinding and heat treat are energy and consumable heavy, so a shop-wide 30 percent burden overcharges simple parts and undercharges grinding-intensive ones. Split burden into machine-specific rates where you can: a gear grinder with an expensive dressing habit and a coated-wheel spend carries more overhead per hour than a manual assembly bench. This is also where warranty exposure belongs. Power Transmission Warranty Return Rate lets you convert a field return rate into a per-unit reserve, and on gearboxes a 1 to 2 percent return rate at a $400 replacement plus freight is $4 to $8 per unit that a quote should carry.

Estimates go wrong in predictable places. The most common is quoting a best-case cutting or assembly rate borrowed from a simpler variant, which understates hours across the whole run. The second is pricing scrap at part value instead of accumulated value, which halves the real loss on anything that fails after heat treat. The third is treating machine hours as labor hours, or vice versa, when one operator tends several spindles. The fourth is forgetting changeover: a mixed-ratio order that looks like a clean run actually loses capacity to setups, which Gear Ratio Variant Build Capacity quantifies and which raises effective cost per unit on short batches.

Build the quote bottom-up and pressure-test the total. Sum material, each operation's machine and labor cost at burdened rates, a realistic scrap adder tied to accumulated value, consumables, testing, and a warranty reserve, then apply margin. Cross-check cost per unit against your last actual run of a similar part; if the quote is more than 10 to 15 percent off history, find the driver that moved before you send it. A quote that can be defended line by line survives a customer's cost-breakdown demand and, more importantly, does not surprise you at month-end when the actuals land.

Published 2026-07-01.