PTO shaftCarries the tractor's power to the baler.
PickupIts tines lift the windrow off the ground.
FlywheelA heavy wheel that adds force to each plunger stroke.
Bale chamberThe long box where hay is packed into a bale.
Twine boxHolds the balls of twine that feed the needles.
Plunger (ram)Packs each charge of hay; some manuals call it the ram.
KnottersTie the twine round each finished bale.
NeedlesCarry the twine up through the chamber to the knotters.
Bale chuteSupports the tied bale as it leaves the chamber.
Square baler from above 6
Pickup tinesRows of fingers on bars; some makers call them teeth.
WindguardHolds the windrow down on the tines for even feeding.
Gauge wheelSets how high the pickup tines run above the ground.
Feeder forkMoves crop from the pickup into the bale chamber.
GearboxSealed case of gears; two makers describe hypoid gears in oil.
Slip clutchSlips under overload to protect the gears.
Bale chamber cut open 6
Plunger knifeShears each charge of hay against a fixed knife.
Hay dogsHold the packed hay in place while the plunger returns.
Metering wheel (star wheel)Turned by the passing bale; it trips the knotters.
Bale tension bars (tension rails)Squeeze the bale; more squeeze makes a denser, heavier bale.
Tension springsTightening them raises the pressure on the bale.
Shear boltBreaks under overload before the baler is damaged.
Round baler cut open 9
BeltsRubberized belts roll the hay into a round bale.
TailgateSwings open at the rear to let the bale out.
Round bale chamberGrows with the bale on a variable-chamber baler.
Net wrap systemWraps netting round the bale when it reaches full size.
Bale ramp (bale kicker)Rolls the bale clear and stops it rolling back.
Rotor (feed rotor)Carries crop from the pickup into the bale chamber.
Twine armGuides twine across the turning bale when net is not used.
Take-up arm (belt tension arm)Moves as the bale grows and keeps the belts tight.
Starter rollSupports the core as the bale starts to form.
How this list was compiled: Part names and every detail line were checked, for facts only and with nothing copied, against pages read on 2026-09-20. Wikipedia: https://en.wikipedia.org/wiki/Baler (tines in the pickup lift the windrow; in offset balers the bale chamber runs the length of one side, usually the left seen from the rear; a combined plunger and knife work at the front of the chamber; a heavy flywheel adds force to the plunger; a spiked measuring wheel turned by the emerging bale trips the knotters; two, sometimes three, strands of twine; a thrower or kicker as an option; on a round baler rubberized belts, fixed rollers or both, netting or twine wrapped when the bale reaches a set size, the back of the baler swinging open to discharge the bale, and variable-chamber balers). Two operator's manuals for small square balers, read in full text at the Internet Archive: https://archive.org/details/allis-chalmers-303-bailer-operator-manual (baler tongue to tractor drawbar; front and rear universal joints; PTO slip clutch that protects the gears; oil bath gear box with hypoid gears; shear bolt in the flywheel; pick-up teeth on tooth bars; feed rakes; baling ram whose knife works against a stationary knife; haydogs and hay retainer that hold the crop while the ram moves forward; star wheel that trips the knotters; twine box, twine guides and the eye of each needle; ram safety stop that protects the needles; compression bars, springs and cranks at the rear of the bale chamber that set bale weight; bale chute; pick-up gauge wheel, bale thrower and wagon hitch as options) and https://archive.org/details/manualzilla-id-5925331 (drawbar, cardan shaft, clutch and safety bolt on the flywheel, feed forks, piston with piston knife and fixed knife, needles that bring the twine to the knotters through the top of the bale chamber, needles timed with the piston, metering wheel with trip lever, compression springs, bale unloading platform). Two makers' product pages for small square balers: https://agriculture.newholland.com/en-us/nar/products/haytools-spreaders/hayliner-small-square-balers (floating windguard above the tines, feeder fork and packer fork, six hay dogs that hold charges in place, bale case, tension springs, tension doors and side tension rails, twine box with a hinged lid, bale chute, pneumatic gauge wheel, flywheel, hypoid gearbox, slip clutch, shearbolt, knotters) and https://www.masseyferguson.com/en_us/products/hay-and-forage/square-balers/mf-1800.html (flywheel, gearbox, pick-up teeth, plunger, stuffer forks, bale chamber, knotters; its pre-compression chamber belongs to an in-line design and is not drawn). Round balers, two makers' product pages: https://agriculture.newholland.com/en-us/nar/products/haytools-spreaders/new-roll-belt-round-balers (belts, starter roll that supports the core, take-up arm that moves as the bale grows, net wrap applied from the front of the baler, dual twine arms, tailgate, windguard, gauge wheels, a sprung bale ramp that rolls the bale clear and stops it rolling back) and https://www.kuhn-usa.com/hay-forage/balers/round-balers/vb-3100 (belt tensioning arm, net fed in at the bale chamber's front, double twine arm, feeding rotor, starter roller, bale kicker, pickup gauge wheels, pickup windguard). Round balers, two operator's manuals at the Internet Archive: https://archive.org/details/manualzilla-id-5637045 (tailgate, also called hatch, with a safety lock device; net binder; ejector; pick-up wheels that set the working height so that the teeth never touch the ground) and https://archive.org/details/manualzilla-id-5806049 (rear door with safety stops, twine arm, net binding device, tine ground height set by the pick-up wheel). Two extension safety articles, read for names only: https://extension.psu.edu/safely-making-and-handling-large-hay-bales and https://ag-safety.extension.org/safety-recommendations-when-baling-and-handling-round-bales/ (PTO shields, tongue, belts, bale chamber, lock-out devices for the open tailgate). Makers appear here as sources only and no maker is named on the page. A part is asked only when at least two of these sources show it. Read again on 2026-09-30, with these added for the names the first sources gave only once: Hay and Forage Harvesting, Deere & Company, Fundamentals of Machine Operation, searched in full text at https://archive.org/details/hayforageharvest0000ride (plunger knife and stationary knife, hay dogs, bale tension bars, slip clutch, pickup gauge wheel, metering wheel, twine box, take-up arm, tailgate); a second operator's manual for a small square baler, https://archive.org/details/Ford2505041HayBalerOwnersManual (plunger knife, bale tension bars, hay dogs); https://archive.org/details/international_harvester_mccormick_number_46_baler_operators_manual (plunger knife); a parts catalog, https://archive.org/details/New_Idea_551_Rectangular_Baler_Parts_Catalog_ (feeder fork); and two farm magazines for the round baler's ramp, https://archive.org/details/sim_farm-industry-news_mid-march-1993_26_5 and https://archive.org/details/sim_american-agriculturist_2003-05_200_5 (bale ramp), with https://archive.org/details/sim_minnesota-history_farm-show_1987_11_5 (bale kicker). Licence: own work
The drawing: Drawn for this site on 2026-10-05 by script, from plain geometry: composite teaching machines, and no maker's machine is shown. Nothing is traced or copied. The forms were checked by eye against photographs of real balers on Wikimedia Commons (small square balers, round balers and a pickup close up). Licence: own work
How it works
The parts of a baler are easiest to learn in the order the hay meets them: pickup, feeder fork, plunger, needles, knotters, chute. Three drawings show a small square baler from above, its bale chamber cut open, and a round baler cut open.
Read the full guide
How hay moves through a small square baler
The pickup tines lift the windrow while the windguard holds it down. The feeder fork moves each charge into the bale chamber. There the plunger packs it, and the plunger knife shears it off against a fixed knife. Hay dogs hold the packed hay while the plunger returns. The bale creeps rearward between the bale tension bars and leaves over the bale chute.
Where the needles and knotters sit
The knotters sit on top of the bale chamber and the needles wait underneath it. The passing bale turns the metering wheel. At full length the wheel trips the knotters, the needles carry the twine up through the chamber, and the knotters tie it. Needles and plunger are timed so that the two never meet.
Start here: 10 main parts walks the square baler from tongue to bale chute before the harder names, such as hay dogs and metering wheel.
Typed answers take the names other manuals use: ram for plunger, star wheel for metering wheel, teeth for tines, rear door for tailgate.
Questions and answers
What are the main parts of a hay baler?
A small square baler has a tongue and a PTO shaft at the front, a pickup and feeder fork that gather the hay, a plunger that packs it in the bale chamber, needles and knotters that tie it, and a bale chute at the rear.
What does the knotter do on a baler?
Each knotter ties one strand of twine round the finished bale. The metering wheel trips the knotters when the bale is long enough, and the needles bring the twine up to them. A small square bale carries two strands, sometimes three.
What are hay dogs on a baler?
Hay dogs are small catches in the wall of the bale chamber. They hold each packed charge of hay in place while the plunger moves forward for the next one. The drawing shows one in the top wall and one in the bottom wall.
What is the difference between a square baler and a round baler?
A square baler packs hay with a plunger in a straight chamber and ties it with knotters. A round baler rolls the hay inside belts or rollers, wraps it in net or twine, and opens its tailgate to let the bale out.