Many engineers assume that simply using a larger gear will increase torque — but that’s not always true. This post is written to clear up that confusion and help you make smarter decisions when selecting or sizing gears for torque performance.
A larger gear only increases torque when it changes the gear ratio.
If the ratio stays the same, upsizing alone won’t affect output torque — and in some cases, it can introduce avoidable design inefficiencies.
Learn when gear size affects torque, how gear ratio and layout matter more, and why tooth strength and smarter design often outweigh diameter alone.
Table of Contents
Does a larger gear always increase torque?
No — a larger gear only increases torque when it’s paired with a smaller driving gear to change the ratio.
Torque gain comes from the mechanical advantage between input and output gears, not the gear’s size in isolation. If the ratio doesn’t change, simply upsizing a gear won’t increase torque.
We often see this misunderstanding in motion control projects — teams add a larger gear to the output shaft thinking it will deliver more torque, but without changing the driver, nothing improves. For example, switching from a 40:20 gear pair to an 80:40 still gives you a 2:1 ratio. You’ve doubled the diameter, but torque output remains the same — along with increased part cost, larger housings, and higher inertia.
This assumption can lead to overbuilt assemblies, longer machining cycles, and unnecessary stock adjustments — especially in CNC contexts where gear bores, keyways, and face widths all add complexity at larger sizes.
Design Takeaway:
Only increase gear size when it changes the input-to-output ratio. If torque is falling short, consider adjusting the ratio, repositioning shafts, or using compound stages — all of which we can help you evaluate without resorting to oversized components that raise cost and delay production.
What increases torque more — gear size or gear ratio?
Gear ratio increases torque — not gear size alone.
Torque gain comes from the relationship between input and output gears. A larger gear won’t deliver more torque unless it increases the ratio by being paired with a smaller driver.
We’ve reviewed designs where engineers enlarged the output gear, expecting a torque increase — but the ratio stayed the same. For example, both a 60:20 and a 120:40 gear pair deliver a 3:1 ratio. Same torque gain, double the footprint — and double the machining cost for CNC-bored hubs, keyways, and housing space.
If you’re working with motor-driven gearboxes, compact reducers, or transmission stages, always think in relative size (gear ratio), not absolute diameter. Upsizing parts without ratio gain often leads to heavier components, more inertia, and longer machining lead times — with no performance benefit.
Design Takeaway:
Optimize gear ratio before increasing part size. We help teams eliminate unnecessary oversizing by evaluating whether torque gains can come from smaller driver gears instead — preserving envelope limits while lowering cost.
How does gear ratio affect torque and speed?
Gear ratio increases torque by the same factor it reduces output speed.
A 3:1 gear ratio gives you 3× more torque, but the output spins ⅓ as fast. It’s a direct trade-off: more force, less speed.
This balance becomes critical when designing motor gearboxes or motion systems where both torque and RPM matter. For example, we’ve supported actuator designs where torque requirements pushed teams toward a 4:1 reduction — but that ratio dropped output speed so far that response time fell below acceptable thresholds. We helped adjust to a 3:1 setup with a reinforced shaft instead, recovering speed while still meeting load demands.
Here’s a quick reference for common gear ratio effects:
Gear Ratio Torque Multiplier Speed Reduction
2:1 2× torque ½ speed
3:1 3× torque ⅓ speed
4:1 4× torque ¼ speed
Design Takeaway:
Every gain in torque comes with a proportional loss in speed. If your output RPM and torque targets are conflicting, we can help assess whether ratio, motor specs, or mechanical reinforcement should be adjusted — before production locks in an overconstrained design.
Is it better to use a larger gear or change the ratio?
Changing the gear ratio is almost always more efficient than using a larger gear.
Ratio affects torque directly, while upsizing a gear adds mass, inertia, and manufacturing cost — often with no additional benefit.
We’ve reviewed designs where engineers tried to boost torque by increasing the driven gear from 60 mm to 90 mm, leaving the 30 mm driver unchanged. While that technically improved the ratio from 2:1 to 3:1, the same result could’ve been achieved by shrinking the driver from 30 mm to 15 mm — with far less cost, weight, or packaging impact.
Here’s a quick comparison:
Option Torque Result Impact
Increase driven gear size ✅ Increases torque ❌ Larger housing, more material, longer machining
Decrease driver gear size ✅ Increases torque ✅ Smaller parts, easier packaging, lower cost
We’ve helped teams re-spec input gears to achieve torque goals while keeping outer dimensions fixed — avoiding unnecessary redesigns to the enclosure or mounting structure.
Design Takeaway:
Start with the ratio, not the size. If you’re hitting torque limits, we can help explore whether downsizing the driver gear delivers the gain — without triggering ripple effects in BOM cost, lead time, or housing clearance.
Designing for torque — gear size concerns?
We help size gears for real-world load • Confirm torque without overbuilding
Does gear placement (input vs output) affect torque?
Yes — the larger gear must be on the output shaft to multiply torque.
Placing the large gear on the input reverses the effect, reducing torque instead of increasing it — a common cause of underperformance in geartrain designs.
We’ve seen teams correctly calculate gear ratios (e.g., 3:1), but mount the larger gear on the motor shaft rather than the driven shaft. That flipped the mechanical advantage — resulting in reduced torque, stalling under load, and the need to rush rework. Fixing it didn’t require new parts, just reversing gear roles and revalidating the shaft fits.
Beyond torque behavior, incorrect gear placement can:
- Increase motor-side backlash and inertia
- Stress bearings due to uneven load transfer
- Mismatch torque ratings across components
This is especially risky in systems with deceleration loads or frequent reversals — such as actuators, servo drives, or belt-fed drives with uneven tension.
Design Takeaway:
Always place the larger gear on the driven side if torque increase is your goal. Before modifying specs or ordering new parts, we can help verify placement and tolerances — preventing costly misalignment and unexpected drivetrain wear.
Which gear in a pair gains torque?
The driven (output) gear gains torque — the driver gear only transmits it.
Torque increases on the gear that’s being driven — not the one doing the driving. The input gear sees a reaction load, but the output gear must absorb the full multiplied torque.
We’ve supported assemblies where teams incorrectly reinforced the input shaft — thinking that’s where the torque lands. In reality, the larger, driven gear receives the amplified force. This affects shaft diameter, key slot depth, hub material, and even surface finish requirements for torque transmission.
A common failure mode: output keys shearing during load spikes because only the motor-side shaft was reinforced.
Here’s a simplified view:
Gear Role Torque Behavior Design Focus
Driver (input) Applies force Must resist reaction torque and maintain speed
Driven (output) Gains torque Must withstand multiplied torque & load
Design Takeaway:
Only the driven gear experiences torque amplification. Always reinforce the output-side shafting and interfaces — and we can help evaluate whether your keys, bores, and press fits are properly sized before sourcing or machining.
How can I increase torque without upsizing the gear?
Increase torque without upsizing by adjusting ratios, adding gear stages, or improving materials.
If you’re space-constrained or cost-sensitive, you can boost torque without enlarging any parts — by redesigning the power path itself.
Here are smart alternatives we’ve implemented:
Method Torque Gain Impact on Design
Change input gear size ✅ High Maintains same housing, better efficiency
Add compound stage ✅ Very High Increases total gear count, but keeps parts compact
Use stronger materials ✅ Moderate Improves tooth load without size change
Improve tooth geometry ✅ Moderate Requires design/DFM update, not size increase
In one servo assembly, we replaced a large gear that couldn’t fit with a two-stage nested system using 4140 hardened steel — achieving higher torque and lower inertia. This avoided tooling changes and kept production under 50 μm flatness across both plates.
Design Takeaway:
Torque doesn’t require bigger parts. We can help you hit load specs using smarter layout, stronger materials, or more efficient gearing — without forcing major changes to your envelope or BOM.
Does gear diameter matter if the ratio stays the same?
No — if the gear ratio stays the same, increasing diameter alone does not increase torque.
Gear diameter only matters relative to the other gear. If both gears are scaled up equally, torque output remains unchanged.
For example, a 60:20 gear pair gives a 3:1 ratio. A 120:40 pair still gives a 3:1 ratio — same torque, just more mass and cost. We’ve seen teams increase both gears assuming they’d boost performance, only to end up with heavier parts, more inertia, and higher machining time — for zero mechanical advantage.
This often happens when teams think larger = stronger — which is only true when the ratio changes or when tooth strength and surface engagement are improved.
Design Takeaway:
If your gear ratio is locked, scaling up gear diameter just adds material and expense. We help teams validate whether any size increase is justified — or if a smarter, leaner design hits the same torque target more efficiently.
Does gear diameter matter if the ratio stays the same?
No — if the gear ratio remains constant, increasing both gear diameters does not increase torque.
Torque gain depends on the relative size of the driver and driven gears — not their absolute diameters. Scaling both gears up by the same factor changes nothing except cost, weight, and footprint.
We’ve reviewed designs where a team replaced a 60:20 gear pair with a 120:40 pair — thinking the larger size would increase torque. But the ratio stayed at 3:1. The torque output was unchanged, while the parts became heavier, the housing had to be reworked, and raw material cost increased by over 30%.
This is a common trap in early design: larger feels “stronger,” but without a ratio shift, there’s no mechanical advantage. Unless you’re gaining tooth strength, increasing contact area, or engaging more teeth per load cycle, scaling up both gears simply adds machining time and inertia.
There are situations where a larger gear is needed — to reduce tooth stress or avoid undercutting at small pitch diameters — but these are geometry-driven constraints, not torque optimizations.
Design Takeaway:
Unless you’re addressing a strength or clearance issue, don’t increase gear diameter without adjusting the ratio. We help teams validate whether size increases actually add performance — or just add cost. In most cases, smarter ratios or better material choices solve the problem more efficiently.
Conclusion
Torque doesn’t come from size alone — it comes from smart design choices. We help engineers optimize gear-driven assemblies without overbuilding. Contact us to explore manufacturing solutions tailored to your product’s torque, space, and cost requirements.
Frequently Asked Questions
Yes — clearly specifying fillet radii or chamfers reduces quoting ambiguity. If left undefined, shops will often apply defaults (e.g., R0.5 mm), which may not match your mating part. Chamfers also ease assembly for press fits. We can advise standard callouts that speed up machining and prevent mating misfits.
Not always. ISO 2768-m may be fine for non-critical features, but for gear bores handling torque, a specific H7 or H6 tolerance ensures proper press or sliding fit. We help identify which features actually need tight tolerances — and which can remain general to reduce inspection cost.
Specify width, depth, and key fit tolerance. Avoid just calling “DIN6885” unless the entire shaft system follows that standard. For splines, include profile (involute or straight-sided), number of teeth, and fit type (interference, sliding). We assist in translating these details into clean 2D drawings ready for quoting.
Yes — once your gear OD exceeds ~150 mm, many shops need to switch from standard vises to soft jaws or faceplate fixturing. This can raise setup time and affect pricing for low volumes. If you’re scaling up, we help pre-check fixture fit and recommend geometry simplifications.
Common delays include:
- Missing center bore spec or pilot hole
- Incompatible tolerances across mating features
- Undefined tooth surface finish (Ra)
- Tapped holes too close to internal edges
We often review models to flag these before release — saving time on clarification emails or reprogramming.
Deep blind bores, thin wall hubs, and undercut shoulders are three major cycle-time drivers. If your bore depth exceeds 2.5× diameter, expect slower passes or special tooling. Undercuts often require custom inserts. We help optimize gear hub designs by balancing strength with tool access — avoiding unnecessary setups.
