Spline Count and Size Guide: 1-3/8″ vs 1-3/4″ PTO Connections

A Power Take-Off (PTO) shaft is the rotating mechanical link between a tractor or prime mover and the implement or machine it drives. At the core of every PTO connection is a splined stub — a cylindrical section machined with parallel ridges called splines — that engages with a matching socket on the driven equipment. This interference-fit geometry transmits torque reliably even under extreme rotational and angular loads. The two dimensions that define spline compatibility are the outside diameter of the stub and the number of splines cut into it. Get either dimension wrong, and the connection either will not engage at all or will engage loosely and fail within hours.

In the UK agricultural and industrial market, two configurations dominate almost every application encountered in the field: the 1-3/8″ (34.9 mm) stub diameter and the 1-3/4″ (44.5 mm) stub diameter. Within each diameter category, further variation exists in spline count — principally 6 splines and 21 splines for the 1-3/8″ size, and 20 splines for the 1-3/4″ size. Understanding which combination your tractor PTO output shaft uses, and which standard your implement demands, is not optional engineering knowledge — it is a fundamental prerequisite for safe, efficient power transmission.

The engineering elegance of a splined PTO connection lies in its distributed load-bearing geometry. Rather than relying on a single keyway or friction-fit collar, the splined stub transfers torque across every tooth simultaneously. When a 1-3/8″ six-spline stub engages its socket, the torque path flows radially outward through six discrete contact patches. Each patch carries a proportional share of the total applied torque, which means the contact stress on any individual spline tooth stays within the fatigue limit of the base material — typically a medium-carbon alloy steel such as 40Cr or 42CrMo — provided the shaft is not overloaded. The same geometric logic explains why the 1-3/8″ twenty-one-spline variant can handle roughly double the torque of its six-spline counterpart despite sharing an identical external diameter: distributing the load across 21 teeth instead of 6 reduces peak contact stress by a factor of approximately 3.5, which dramatically extends service life under cyclic loading conditions.

The 1-3/4″ twenty-spline connection takes this principle further by simultaneously increasing both the tooth count and the pitch radius — the distance from the shaft centreline to the mid-point of each spline tooth. Because torque is the product of force and radius, a larger pitch radius means the same tangential force generates proportionally more torque capacity. This is the physical reason that 1-3/4″ PTO connections are mandated on high-horsepower tractors and heavy implements: they are not merely a larger version of the smaller standard but a geometrically superior solution for high-power density applications such as large rotary mowers, industrial wood chippers and grain augers common in Yorkshire and East Anglian arable operations.

☰ Splined Stub

Medium-carbon chromium-molybdenum alloy steel (42CrMo4 to EN 10083-3 or equivalent). Through-hardened then induction-hardened at the spline teeth to HRC 56–62. Provides fatigue resistance at the stress concentration roots of the spline teeth — the most common initiation point for spline fracture under shock loads.

■ Outer Tube

Cold-drawn seamless steel tube (DOM) to BS EN 10305-1. The overlapping square or hexagonal telescoping profile maintains full engagement over a wide range of implement working distances. ERW tube sections, where used in lower-cost variants, must pass magnetic particle inspection to detect seam weld discontinuities that can propagate under bending fatigue.

◆ Universal Joints

Forged medium-carbon steel yokes with hardened needle-roller bearing kits retained in precision-ground bearing cups. The grease nipple arrangement enables in-situ lubrication without shaft removal — a critical serviceability feature for farm operations during busy harvest windows. Bearing cup case depth is tightly controlled to maintain correct needle-length engagement.

▶ Safety Guard

High-density polyethylene (HDPE) or UV-stabilised polypropylene rotating shield. Must comply with BS EN ISO 11684-2 for rotating shaft guarding. The guard assembly is mounted on sealed bearings that allow it to remain stationary while the shaft rotates at up to 1,000 rpm, preventing the entanglement hazard that has historically caused serious injuries on UK agricultural holdings.

Precision Spline Tolerancing

Spline tooth profiles are hobbed and shaved to DIN 5480 or ISO 4156 tolerances. Tight fit-class tolerances minimise backlash and prevent the fretting corrosion that typically initiates at the spline tooth flank under oscillating torque reversal.

Overload Protection Integration

Friction-disc, ratchet and shear-bolt torque limiters can be integrated directly into the PTO shaft assembly at the driveline design stage. This removes expensive secondary drivetrain damage caused by implement jamming — a common occurrence when arable chippers encounter embedded fieldstones in the clay soils of the East Midlands.

Wide Telescoping Range

Profiled square or hexagonal inner-outer tube pairs provide a telescoping range of 200 mm to 600 mm across standard catalogue lengths. This range accommodates the widely varying hitch-to-gearbox distances encountered across different tractor models commonly operated on UK mixed farms.

Quick-Release Collar Design

Spring-loaded collar release at the PTO stub end allows single-handed implement connection and disconnection without tools. The push-button retention mechanism maintains positive axial lock under all normal operating conditions whilst permitting rapid changeover between implements — a practical requirement for UK contractors operating multiple machines in a single working day.

Corrosion-Resistant Finish

Zinc phosphate conversion coating followed by agricultural-grade enamel topcoat on outer profiles, with grease-filled internal profiles to resist moisture ingress. This coating system is specified to withstand a minimum of 240 hours salt-spray testing to BS EN ISO 9227, ensuring adequate corrosion resistance in the wet West Country and upland Scottish operating environments.

Dynamic Balance Certification

High-speed PTO shafts operating at 1,000 rpm are dynamically balanced to ISO 1940-1 Grade G6.3 or better. A residual imbalance exceeding this threshold at 1,000 rpm generates a centrifugal force that induces vibration transferable to the tractor cab — both a driver comfort issue and a durability concern for bearing life in the tractor’s output gearbox.

PTO shaft connections in the 1-3/8″ and 1-3/4″ categories serve a remarkably wide spectrum of industries across the UK. In the arable counties of Lincolnshire, Cambridgeshire and East Yorkshire, the six-spline 1-3/8″ connection remains the standard for lighter equipment — fertiliser spreaders, compact rotary mowers, small seeding units and slurry pumps rated below approximately 70 hp. These connections are straightforward to service, and replacement stubs and sockets are held in stock by most agricultural merchants in market towns across the region, making same-day field repair a realistic expectation.

The twenty-one-spline 1-3/8″ variant occupies a commercially important middle ground that is often misunderstood. Its external dimensions are identical to the six-spline version, which creates a dangerous assumption among operators that the connections are interchangeable — they are absolutely not. The twenty-one-spline standard carries roughly twice the torque capacity and was developed specifically to address failures on mid-power tractors equipped with 1,000 rpm PTO outputs where six-spline connections were consistently failing under the sustained high-speed loads imposed by large silage mowers and cultivators in Yorkshire Dales livestock operations.

PTO shafts in heavy agricultural and industrial use across UK operations

🌿 Agriculture (UK Arable & Livestock)

Grain augers, combine attachment drives, potato harvesters, large round balers, manure spreaders, strip-till cultivators. The 1-3/4″ standard is mandatory on machines requiring above 80 hp PTO input — the norm in modern arable operations in the Vale of York.

🏠 Construction & Civil Engineering

Hydraulic breakers, compactor drives, concrete mixers, auger boring rigs and trenching machines. Birmingham’s construction supply chain and infrastructure projects across the West Midlands have driven sustained demand for heavy 1-3/4″ PTO-driven hydraulic pump assemblies.

⚒ Forestry & Woodland Management

PTO-driven wood chippers and log splitters operating under peak shock loads of 3–5 times rated torque. The 1-3/4″ twenty-spline connection is the only appropriate standard for chippers processing timber above 200 mm diameter. Friction-disc overload limiters are a mandatory safety addition for forestry chipper drivelines.

⛏ Industrial Manufacturing

Steel processing conveyors, pump drives in chemical plants, mixer drives in food processing facilities. Sheffield’s remaining heavy manufacturing operations and the food processing industry in Lincolnshire and Humberside rely on custom-length PTO shaft assemblies for machine drives where standard catalogue lengths are insufficient.

🚢 Grounds Maintenance & Municipal

Highway verge mowers, sports ground aeration equipment, tractor-drawn sweepers and winter-gritting machines operated by local councils across Scotland, Wales and Northern Ireland. These applications typically use 1-3/8″ six-spline or twenty-one-spline connections depending on the carrier vehicle horsepower classification.

Ever Power CNC machining centre — precision spline manufacturing

Bridgeford’s engineering manager contacted Ever Power after discovering the company’s technical documentation through an industry referral from a Birmingham-based agricultural machinery distributor. The key issue identified during the initial technical consultation was not shaft overload per se, but a combination of two problems: first, the spline teeth on the original shafts had been hobbed to a loose fit class that allowed fretting movement under oscillating torque; second, the U-joint operating angles were marginally above 15° due to a slight misalignment between the gearbox output flange and the chipper input shaft — an alignment error of under 1.5° that had gone undetected during the original installation commissioning.

Ever Power engineered a custom 1-3/4″ twenty-spline shaft assembly to a tighter DIN 5480 fit class, incorporated dual-axis angle compensation at both U-joint positions to reduce the effective operating angle to 11°, and added a friction-disc torque limiter set to 2,800 Nm — giving a safety margin of 20% below the spline assembly’s rated capacity while allowing normal peak chipper drum impacts to pass through. After installation, Bridgeford ran the line continuously for six months without a single drivetrain-related shutdown, saving an estimated £24,000 in avoided lost production and £6,500 in replacement shaft costs compared with the previous year’s failure rate. The engineering team has since standardised on Ever Power custom PTO shaft assemblies across the entire processing facility.

“The custom-length 1-3/4″ shaft Ever Power supplied for our Rotherham biomass processing line resolved six months of intermittent spline failures in one installation. The DIN tolerance documentation and hardness certificates they provided with the delivery were exactly what our compliance team needed. We have not had a single drivetrain fault in the nine months since fitting.”

— D. Hartley, Plant Engineering Manager · Bridgeford Processing Ltd, Sheffield

“We manage a large grain farm in Lincolnshire and needed 1-3/8″ twenty-one-spline shafts in a non-catalogue length for our wide-working tillers. Ever Power turned around a custom order in twelve working days, including full material certification. The quality is visibly superior to what we had before — the spline fit is noticeably tighter and the grease nipple arrangement makes winter maintenance straightforward.”

— T. Marsden, Farm Director · Marsden Arable Partnership, Spalding, Lincolnshire

“Our Birmingham-based construction hire business runs a fleet of PTO-driven auger boring rigs. Reliability during active contracts is non-negotiable. Ever Power’s stock of 1-3/4″ assemblies and their rapid despatch to the West Midlands has meant we have been able to keep every rig running through the busiest contracting months. The technical support on sizing and overload protection was genuinely expert — not just catalogue reading.”

— P. Whitfield, Operations Director · Westmid Groundworks Equipment Ltd, Birmingham

Step 01

Measure the PTO Output Stub

Use a Vernier calliper to measure the external diameter of the tractor or prime mover output stub. A reading of approximately 34.9 mm indicates the 1-3/8″ standard; a reading of approximately 44.5 mm indicates 1-3/4″. Then count the spline teeth visible around the circumference to distinguish six-spline from twenty-one-spline at the smaller diameter.

Step 02

Identify the Implement Input Socket

Check the implement manufacturer’s specification plate or operator manual to confirm the input socket diameter and spline count required. If the implement was built to Category II or III three-point linkage, the PTO input will almost invariably follow the corresponding standard — Category II typically pairs with 1-3/8″, Category III and above with 1-3/4″.

Step 03

Calculate Peak Torque Demand

Torque (Nm) = Power (kW) x 9549 / Speed (rpm). For a 100 hp (75 kW) tractor running at 540 rpm PTO speed, the continuous torque is approximately 1,326 Nm. Verify that the selected spline standard’s rated torque exceeds this value with an appropriate safety factor — typically 1.5 for smooth loads, 2.5 to 3.0 for shock-load applications such as chippers and stump grinders.

Step 04

Confirm Operating Speed & Guard Compliance

Verify whether the application runs at 540 rpm or 1,000 rpm. At 1,000 rpm, dynamic balance becomes critical and the guard must comply with the relevant BS EN safety standard. Confirm that the guard assembly supplied rotates freely on its own bearings, that the profile of the guard does not limit the required working angle, and that it is correctly secured against axial travel.

Critical Warning: Adapter Use and Non-Compliant Cross-Connections

Adapters that convert a 1-3/8″ six-spline stub to a 1-3/4″ socket — or vice versa — are commercially available but should only be used after explicit torque capacity verification against the adapted combination’s lowest-rated component. In most cases, the adapter itself becomes the weak link in the driveline. Use of adapters may also invalidate the CE marking of the implement and the operator’s liability position under the UK Provision and Use of Work Equipment Regulations (PUWER) 1998. When in doubt, specify a shaft assembly built correctly to both end connections from the outset.