A trans-axle transfers the power of a lawnmower engine into forward and backward motion. It sounds like a made-up word, and I suppose it is. It’s a cross between the words transmission and axle.
So what is a lawn mower transaxle? A mower trans-axle is a single mechanical assembly that combines the functions of transmission, differential, and axle. The word trans-axle describes both the type of transmission and the axle set up in a vehicle.
Combining all these separate components into one integrated unit was a stroke of genius, and in this post, you’ll find out why it’s so clever.
What’s A Transaxle?
Before the trans-axle was invented, the transmission, differential, and driving axles were all separate assemblies. All these separate components do a different job (more on that later) and were as you can imagine quite bulky, getting them to fit into a small area was a challenge.
The trans-axle is compact and combines all the functions of the separate assemblies, its main function is to transfer engine power into forward motion. Depending on how sophisticated it is, you may have reverse motion and variable speed control. Trans-axles are the normal setup in all front-wheel-drive cars, and most people will refer to a trans-axle as the transmission or transmission assembly.
All lawnmowers will have axles but unless they’re power-driven they won’t have a trans-axle. Most walk-behind self-drive mowers are rear-wheel driven and have the trans-axle mounted at the rear, but some craftsman mowers for example are front wheel drive and so the trans-axle is mounted at the front. Rear-drive is better for hilly ground where you need good traction, and front-drive is better for a yard with lots of shrubs and flowerbeds, they’re more maneuverable.
Walk-Behind Mower Trans-axle – Mower trans-axle driven by engine pulley by means of the drive belt.
Higher-end self-drive walk-behind mowers will have variable speed control.
Tractor Trans-axle – A lawn tractor trans-axle is located at the rear of the mower, it’s usually pretty bulky and well defined, with a fat wheel at either side.
Lawn tractor trans-axles are either manual or automatic just like a car. The gear-driven trans-axles have a clutch pedal and a gear selector, the brakes are also mounted on the trans-axle.
The auto version has a variable speed forward and reverse pedal and is usually a hydro-static (oil pressure driven) or CVT type. The highly maneuverable Zero-turn mowers are a little unique in that they have two hydro-static trans-axles, one for each rear wheel. This setup helps them turn tightly, just like a shopping trolley.
Trans-axles are driven as you know by engine power. A belt from the engine pulley, mounted just above the blade, transfers power to the trans-axle pulley. Turning the trans-axle pulley on some basic trans-axle types causes the mower to move, on other more sophisticated types the transaxle pulley spins as soon as the engine starts up and a lever (pedal usually) needs to be operated before the mower actually moves.
Tractor Trans-axle Drive Belt Pulley – Tractor mower transaxle pulley is driven from the engine crankshaft by means of the belt (Kevlar usually).
Common Trans-axle Problems
Mostly they’re pretty durable, belt breaking and wear are common problems on both walk behind and tractor mower trans-axles.
Walk-behind mower transaxles are operated by pulling and holding a lever, the lever is operated by cables, and they need to be adjusted and replaced in time. Nearly all trans-axles are oiled for life, meaning you don’t need to dip or top-up, if fact on most you can’t, there’s no access.
Hydro-static models will need an oil top-up, usually, they’ll have a plastic oil reservoir mounted on top of the trans-axle. A special oil grade is used (20w50) to top up and air will need to be purged from the system, but it’s not difficult to do.
Tractor mower trans-axle brakes will need adjustment, pads may need to be replaced and brakes not working is common. Constant Variable Transmission (CVT) is fitted to some tractors and as they age belt wear and non-OEM fitted belts can cause lots of problems.
What’s A Transmission?
A mower transmission as the word suggests transmits rotational motion (turning of the engine) into linear motion (mower moving forward or backward). Mowers mostly use two types of transmissions – gear driven and hydro-static. The gear-driven type is very basic, the common manual lawn tractor transmission (the manual type usually called a gearbox) will have 5 forward gears and one reverse.
A basic walk behind mower transmission will have one gear speed but the higher end variable speed type gearbox may have a Constant Variable Transmission (CVT) type setup or variable slip.
Gear Driven Transmission with integrated differential
What’s An Axle?
The axle as you know is what the wheels of your mower are fitted to, the axle’s job is to support the weight of the mower, fix the drive assembly firmly to the mower, and hold the wheels in place. The genius of the trans-axle is that the outer case is the axle, it supports the weight of the mower and is fixed solidly to the chassis of the mower. Contained within the axle are all the oily bits, the driveshafts, and the differential.
The differential’s job is to allow one wheel to move independently of the other, this is useful when turning corners as the outer wheels travel further than the inner wheels. The axle structure of a mower doesn’t give much trouble, and apart from removing grass and debris from the cooling fins, it’s maintenance-free.
Axle – It carries the weight of the mower (normally), and enables the wheels to remain fixed to the chassis.
What does a hydro-static mower mean on a riding mower? A hydro-static riding mower refers to the type of transmission fitted. A Hydro-static mower is like an automatic car it operates on oil pressure and progressively pressing a single pedal or lever makes it move faster.
Do I need a riding mower? If your yard is more than 3/4 acre or you are feeling strain pushing a walk-behind mower, then yes you need a riding mower or a robotic mower.
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.