{"id":908,"date":"2026-05-18T06:34:18","date_gmt":"2026-05-18T06:34:18","guid":{"rendered":"https:\/\/foragebaler.com\/?p=908"},"modified":"2026-05-18T06:34:18","modified_gmt":"2026-05-18T06:34:18","slug":"disc-mower-gearbox-maintenance-oil-slip-clutch-blades","status":"publish","type":"post","link":"https:\/\/foragebaler.com\/id\/disc-mower-gearbox-maintenance-oil-slip-clutch-blades\/","title":{"rendered":"Disc Mower Gearbox Maintenance: Oil, Slip Clutch, and Blade Bolts"},"content":{"rendered":"
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<\/div>\n
Mower Mechanical Maintenance Guide<\/span><\/p>\n

Disc Mower Gearbox Maintenance: Oil, Slip Clutch, and Blade Bolts<\/h1>\n

A disc mower’s cutting system spins individual blade discs at 3,000 RPM or higher \u2014 blade tip speeds approaching 150 mph in the most aggressive configurations. At those speeds, a missed oil change in a disc gearbox or a loose blade bolt becomes a high-energy failure event rather than an inconvenience. This guide covers the three maintenance areas where neglect most often ends a mowing day before it should.<\/p>\n

Gearbox Oil Guide<\/a><\/p>\n<\/div>\n<\/div>\n

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<\/p>\n

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Where the Gearboxes Are and What Each Does<\/h2>\n

A disc mower’s drive train typically contains three or four separate gearbox locations, each carrying a different load type and running at a different speed ratio. Operators who think of the “mower gearbox” as a single unit often neglect some of these locations because they are less visible or less intuitively associated with wear. Every gearbox on the mower deserves individual inspection and maintenance.<\/p>\n

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1. Main Input Gearbox<\/div>\n

Receives PTO input (typically 540 RPM) and converts to the higher rotational speed required by the disc bar drive shaft. Houses the largest bevel gear set on the mower. Highest torque load; most susceptible to overheating if oil level is low.<\/p>\n

Oil type: GL-4 or GL-5 gear oil. Check level every 20 mowing hours.<\/div>\n<\/div>\n
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2. Intermediate Disc Gearboxes<\/div>\n

One gearbox unit per disc (typically 3 to 6 on a standard-width mower). Each unit contains a small bevel gear set that drives one disc from the cross shaft. Runs at blade-disc speed (approximately 3,000 RPM at the disc shaft). Highest rotational speed; most sensitive to oil contamination from field debris entry through seals.<\/p>\n

Oil type: per manufacturer specification, often SAE 80W-90. Check all discs annually.<\/div>\n<\/div>\n
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3. Conditioning Unit Gearbox (if equipped)<\/div>\n

On mower-conditioners, a separate gearbox drives the conditioning rolls or flail conditioner at a different speed ratio from the disc bar. Often overlooked in maintenance routines because it is behind the disc bar and less accessible. Runs at intermediate speed; lower load than the disc gearboxes but equally oil-sensitive.<\/p>\n

Oil type: same as main gearbox in most designs. Check separately from disc gearboxes.<\/div>\n<\/div>\n<\/div>\n<\/div>\n

<\/p>\n

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Bevel Gear Oil: The Most Overlooked Disc Mower Maintenance Item<\/h2>\n

\"disc<\/p>\n

In surveys of disc mower failures presented at agricultural dealer service meetings, low or degraded gear oil is identified as the root cause of disc gearbox failure in 45\u201360% of cases. The failure mode is always the same: oil that was never changed or checked allowed contamination buildup; contaminated oil lost its film strength; bevel gear tooth surfaces made metal-to-metal contact; gear teeth progressively pitted and eventually spalled; the gearbox seized or lost structural integrity. A disc gearbox replacement costs $150\u2013$400 per unit. An oil change costs $8\u2013$15 in materials.<\/p>\n

\n\n\n\n\n\n\n\n
Gearbox location<\/th>\nFirst oil change<\/th>\nSubsequent interval<\/th>\nTypical fill volume<\/th>\nCheck level procedure<\/th>\n<\/tr>\n<\/thead>\n
Main input gearbox<\/td>\n50 hours (new machine break-in)<\/td>\nEvery 200 hours or annually<\/td>\n0.5\u20132 liters<\/td>\nRemove level plug (typically on the side face); oil should be at or just below the plug hole<\/td>\n<\/tr>\n
Individual disc gearboxes<\/td>\n50 hours (new machine break-in)<\/td>\nEvery 200 hours or annually<\/td>\n50\u2013150 mL per disc<\/td>\nAccess through the disc mounting cap; small volume \u2014 even 20mL low is significant relative to total capacity<\/td>\n<\/tr>\n
Conditioning unit gearbox<\/td>\n50 hours (new machine break-in)<\/td>\nEvery 200 hours or annually<\/td>\n0.3\u20130.8 liters<\/td>\nLevel plug on conditioner gearbox housing (separate from disc bar gearboxes)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n
Oil condition check:<\/strong> Before the season, drain a small sample from each gearbox (use the drain plug if equipped, or a clean vacuum pump). Fresh GL-5 gear oil is amber to light brown. Contaminated oil appears dark gray (metal particles), black (thermal degradation), or milky (water ingression). Any of these conditions requires immediate oil change and investigation of the contamination source \u2014 a milky appearance means a seal has failed and needs replacement before the gearbox returns to service.<\/div>\n<\/div>\n

<\/p>\n

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The Slip Clutch: Inspection, Rust Seizure, and Spring Tension Setting<\/h2>\n

The PTO input shaft on a disc mower includes a slip clutch \u2014 a torque-limiting device that slips (decouples the drive momentarily) when the cutting system encounters an overload event such as a rock, fence post fragment, or dense windrow buildup. The slip clutch serves the same protective function as the shear bolt on a round baler: it absorbs the overload shock to protect the disc gearboxes and PTO shaft from impact damage.<\/p>\n

Unlike a shear bolt, the slip clutch is reusable \u2014 it slips during the overload and re-engages when the load returns to normal. This makes it convenient but also invisible in its maintenance needs. A slip clutch that has seized from corrosion provides no protection at all, equivalent to having no overload protection device. The most common slip clutch failure mode is not mechanical wear but rust seizure from seasonal storage without activation.<\/p>\n

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Annual Slip Clutch Inspection<\/div>\n
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  1. With PTO shaft disconnected, attempt to rotate the clutch input flange by hand relative to the output flange. A properly functioning slip clutch will slip when you apply 30\u201350 lbs of hand torque at the flange OD.<\/li>\n
  2. If the clutch does not slip at all under hand force, it is seized. Do not apply power until it is freed \u2014 a seized slip clutch transmits the full overload shock directly to the gearbox.<\/li>\n
  3. To free a seized clutch: apply penetrating oil to all friction plate interfaces and allow to soak for 4 hours. Manually actuate by alternating light hammer taps on the clutch plate edges while rotating. This frees surface rust without damaging the friction surfaces.<\/li>\n
  4. After freeing, verify slip torque is within spec by applying a torque wrench to the input flange with the output locked.<\/li>\n<\/ol>\n<\/div>\n
    \n
    Slip Clutch Spring Tension Adjustment<\/div>\n

    Agricultural slip clutches are set by spring preload \u2014 a series of disk springs or coil springs that apply clamping force to the friction plates. Correct preload produces the correct slip torque. Over-adjusted (too much preload) means the clutch transmits higher torque than intended before slipping, reducing overload protection. Under-adjusted clutches slip during normal cutting loads, reducing drive efficiency.<\/p>\n

    How to check:<\/strong> Count the gap between the clutch spring housing faces. The correct gap dimension is specified in the operator’s manual (typically 0.5\u20132mm depending on the clutch model). This gap corresponds to the specified spring preload. Use the adjusting nuts on the tie bolts to set the correct gap, then verify slip behavior with a light manual test.<\/p>\n<\/div>\n<\/div>\n<\/div>\n

    <\/p>\n

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    Blade Bolt Torque: The Safety-Critical Maintenance Item<\/h2>\n

    \"disc<\/p>\n

    A disc mower blade that separates from its mounting disc becomes an unguided projectile \u2014 the blade tip velocity of 100\u2013150 mph converts to a thrown object with enough kinetic energy to penetrate a barn wall at 50 meters. Blade bolt failures due to undertorquing, inadequate retorque, or incorrect bolt specification are a well-documented cause of serious disc mower accidents in the United States. This is not a guideline item \u2014 it is the one disc mower maintenance task that should be treated as non-negotiable.<\/p>\n

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    SAFETY REQUIREMENT \u2014 Blade Bolt Torque Protocol<\/div>\n
    \n
    \u2022 Torque blade bolts to manufacturer specification at each blade change \u2014 not by feel, by a calibrated torque wrench. Typical specifications range from 90 to 180 Nm depending on bolt diameter and disc design. Confirm the exact specification in the operator’s manual for your specific mower model.<\/div>\n
    \u2022 Use only the bolt specification listed for blade mounting. Higher-grade (stronger) bolts are not safer \u2014 they can fail in a different fracture mode that releases the blade more explosively. Never substitute a “stronger” bolt for the specified one.<\/div>\n
    \u2022 Re-torque after the first 2 hours of mowing on new blades or any replaced bolt. Blade vibration during break-in can cause initial seating relaxation that reduces clamp force below specification.<\/div>\n
    \u2022 Replace blade bolts at every blade change. Blade bolts are designed as single-use fasteners that plastically deform during torquing \u2014 a re-used bolt cannot be torqued to original specification and has reduced fatigue life from the prior service cycle.<\/div>\n
    \u2022 Inspect blade bolt heads before each mowing session for cracks, corrosion, or impact damage that reduces the bolt’s structural integrity. Any damaged bolt head requires immediate replacement before the mower enters service.<\/div>\n<\/div>\n<\/div>\n<\/div>\n

    <\/p>\n

    \n

    Disc Bar Inspection: Balance, Blade Wear, and Impact Damage<\/h2>\n

    Each disc on the mower must be dynamically balanced \u2014 the blade pair mounted on it must have matched weights within a tight tolerance. Blade wear and rock impact damage unbalance the disc by changing the mass distribution of one blade relative to its partner. An out-of-balance disc at 3,000 RPM generates vibration forces that accelerate bearing wear in the individual disc gearbox, loosen blade bolts, and create audible resonance that indicates an immediate inspection is needed.<\/p>\n

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    \n
    Blade Replacement \u2014 Match Weight Rule<\/div>\n

    Always replace blades as a paired set on each disc. Replacing one blade of a pair leaves the disc running with mismatched blade masses. The weight difference required to cause problematic vibration at disc speed is less than 5 grams \u2014 about the weight of a coin. Use replacement blades from the same production lot if possible, or weigh both blades before installation and select pairs within 3 grams of each other.<\/p>\n<\/div>\n

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    Blade Wear Inspection<\/div>\n

    Standard disc mower blades are replaceable when: visible length reduction of more than 15mm from original (measure from the pivot hole center to the blade tip); a notch in the cutting edge deeper than 5mm that cannot be removed by sharpening; visible stress cracks at the blade pivot hole (immediate removal \u2014 a blade with a pivot hole crack will fail under operating centrifugal load).<\/p>\n<\/div>\n

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    Post-Rock-Strike Inspection<\/div>\n

    After any rock or metal object strike, stop immediately and inspect all blades on the affected disc and the adjacent discs. A rock strike distorts the blade geometry and may crack the pivot area without visible surface evidence on casual inspection. Run your finger along the full blade surface \u2014 a crack feels like a sharp edge crossing the blade at an angle to the length. Any blade with a crack is immediately non-functional and a safety hazard.<\/p>\n<\/div>\n<\/div>\n<\/div>\n

    <\/p>\n

    \n

    PTO Shaft and Input Protection: Guards, U-Joints, and Drive Angles<\/h2>\n

    The PTO shaft connecting the tractor to the mower input gearbox operates at 540 RPM and transmits the full cutting drive load through two universal joints. Disc mower PTO shafts are exposed to more impact risk than most agricultural equipment PTO shafts \u2014 the cutting action and the high rotational speed of the disc bar create strong vibration inputs that fatigue U-joint bearings and can dislodge PTO shaft guards. For the full specification framework on PTO shaft torque ratings and U-joint service life by shaft diameter, the Spesifikasi komponen gearbox pertanian dan sistem penggerak PTO.<\/a> provide the relevant engineering data.<\/p>\n

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    \n
    U-Joint Inspection<\/div>\n

    With PTO shaft disconnected, grip the shaft and try to move it radially at the U-joint positions. More than 1mm of radial play indicates a worn U-joint bearing. Worn U-joints cause vibration that transmits to the mower input gearbox as a cyclic load spike \u2014 accelerating input shaft bearing wear. Replace U-joint bearing cups at the first sign of play; PTO shaft cross-and-bearing kits are standardized components available from any agricultural supplier for $15\u2013$35 per U-joint position.<\/p>\n<\/div>\n

    \n
    Operating Angle Limit<\/div>\n

    PTO U-joints have a maximum operating angle \u2014 typically 15\u00b0 for standard U-joints and 25\u00b0 for wide-angle joints. Operating beyond this angle creates velocity variation at each revolution (the shaft output speed speeds up and slows down cyclically), which generates torsional vibration in the gearbox. For disc mowers that fold for transport, confirm the PTO shaft is disconnected before folding \u2014 folding with PTO connected can take the U-joint past its operating angle limit, permanently damaging the joint.<\/p>\n<\/div>\n<\/div>\n

    For the mowing quality implications of disc mower vs. sickle bar selection \u2014 including which cutting system has the lower total driveline maintenance burden \u2014 the disc mower vs sickle bar comparison<\/a> covers the full equipment decision framework. The mowing speed, conditioning intensity, and field condition factors that determine how much load the disc gearboxes carry per operating hour are analyzed in the panduan memotong dan merawat rumput<\/a>.<\/p>\n<\/div>\n

    <\/p>\n

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    Annual Service Schedule: What to Do and When<\/h2>\n

    \"mower<\/p>\n

    \n\n\n\n\n\n\n\n\n\n\n
    Service item<\/th>\nSelang<\/th>\nProcedure<\/th>\nConsequence of skipping<\/th>\n<\/tr>\n<\/thead>\n
    All gearbox oil change<\/td>\n200 hrs \/ annually<\/td>\nDrain all gearboxes warm; flush with clean oil before refilling; use OEM-specified gear oil grade<\/td>\nGearbox failure within 1\u20133 seasons<\/td>\n<\/tr>\n
    Slip clutch activation test<\/td>\nPre-season annually<\/td>\nManual slip test; free if seized; check spring gap<\/td>\nNo overload protection; catastrophic gearbox damage on first rock strike<\/td>\n<\/tr>\n
    All blade bolt replacement<\/td>\nEvery blade change<\/td>\nUse new OEM-spec bolts; torque wrench to specification; re-torque after 2 hours<\/td>\nBlade separation \u2014 serious safety hazard<\/td>\n<\/tr>\n
    U-joint inspection<\/td>\nPre-season annually<\/td>\nRadial play test; replace bearing cups with play >1mm<\/td>\nVibration-accelerated gearbox bearing wear; eventual U-joint failure<\/td>\n<\/tr>\n
    All grease zerks<\/td>\nEvery 8 hrs mowing<\/td>\nGrease all disc pivots, skid shoe pivots, cutter bar hinge points<\/td>\nPivot wear; increased maintenance frequency<\/td>\n<\/tr>\n
    Blade inspection and rotation<\/td>\nEvery 50 hrs \/ after rock strikes<\/td>\nVisual inspection for cracks, length reduction, edge condition; rotate worn blades if reversible type<\/td>\nCutting quality decline; unbalanced disc vibration<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

    <\/p>\n

    \n

    Disc Mower Maintenance FAQs<\/h2>\n
    \n
    \nMy disc mower vibrates during the first few minutes of operation but smooths out when it warms up. What is causing this?+<\/span><\/summary>\n
    Vibration that improves as the machine warms up is almost always related to gearbox oil viscosity in cold conditions. Cold gear oil (below 40\u00b0F) has significantly higher viscosity than at operating temperature \u2014 the thick oil does not distribute evenly through the gearbox at startup, creating momentary imbalanced lubrication. This is most pronounced in the individual disc gearboxes where oil volume is very small (50\u2013150mL) and the bevel gears spin at high speed. The vibration resolves as the oil reaches operating temperature and proper viscosity. If the vibration does not resolve within 3\u20135 minutes of operation, or if it is accompanied by a grinding sound, the cause is more likely a worn bearing or misaligned gear \u2014 stop and inspect before continuing. Cold-start vibration can be reduced by using a gear oil specification with lower pour point (ISO VG 68 instead of VG 90 for cold-climate operations).<\/div>\n<\/details>\n
    \nCan I sharpen disc mower blades, or must I always replace them?+<\/span><\/summary>\n
    Standard pivoting disc mower blades can be sharpened 2\u20133 times before they reach the minimum length replacement threshold. Sharpening is performed with a bench grinder or angle grinder on the flat face of the blade (the face opposite the beveled cutting edge) \u2014 never grind the beveled face, as this changes the cutting geometry and can create stress concentration points at the newly ground surface. The blade should be sharpened symmetrically on both edges to maintain weight balance. Weigh the blade after sharpening and compare to the matched partner blade \u2014 if the pair is more than 3 grams different after sharpening, they should be replaced as a set rather than resharpened to matching weight, as removing metal to match weights reduces blade length and may bring the shorter blade below the minimum safe operating length. Non-reversible blades that have been sharpened to minimum length (typically indicated by the manufacturer’s wear indicator mark) must be replaced, not further sharpened.<\/div>\n<\/details>\n
    \nOne of my disc gearboxes runs noticeably hotter than the others at the same operating conditions. What does this mean?+<\/span><\/summary>\n
    A disc gearbox running significantly hotter than its neighbors (more than 30\u00b0F above the others when measured by infrared thermometer after 30 minutes of mowing) is the most reliable early warning sign of impending gearbox failure. The elevated temperature indicates increased friction from one of three causes: oil level low in that gearbox specifically (check level immediately); oil contaminated in that unit (seal failure allowing soil or water ingress); or bearing beginning to fail within that gearbox. The diagnostic sequence: check oil level and condition first (5 minutes); if oil is correct, plan replacement of that gearbox unit before its next extended use \u2014 a hot gearbox has already progressed to the point where failure is likely within 10\u201330 hours of additional operation. Continuing to operate a hot gearbox hoping it will “sort itself out” almost always results in complete gear failure that destroys the housing and requires a significantly more expensive repair.<\/div>\n<\/details>\n
    \nHow do I know if my slip clutch is slipping during normal cutting instead of only on overloads?+<\/span><\/summary>\n
    A slip clutch that is slipping during normal cutting shows itself through two symptoms: reduced cutting performance (the disc bar is not maintaining full speed because the clutch is dissipating drive energy through slippage), and heat \u2014 a slipping clutch generates significant friction heat that you can detect as a burning smell from the PTO area or elevated temperature at the clutch housing. A properly set slip clutch should not slip at all during normal cutting conditions in standing hay or light alfalfa. If you can induce slippage by starting in a dense windrow or during initial engagement from idle, the spring preload is set too low. Tighten the adjusting nuts on the tie bolts until the gap dimension matches the specification, then test again. If the clutch still slips in moderate conditions after adjustment to specification, the friction plates may be glazed from previous overuse and need replacement.<\/div>\n<\/details>\n
    \nWhat is the correct oil level check procedure for the individual disc gearboxes?+<\/span><\/summary>\n
    Individual disc gearboxes on towed mowers are most accurately checked with the mower on level ground and the disc bar in its operating position (not folded for transport). The oil level plug on each individual disc gearbox is typically accessible by removing the disc blade assembly (or through a separate plug hole on the gearbox housing). The oil should be at or just below the level plug hole when the plug is removed. Because individual disc gearboxes are very small (50\u2013150mL total capacity), even a 20mL oil loss represents 10\u201330% of total capacity \u2014 a significant reduction in the oil film on gear tooth contact surfaces. On new machines, check all individual disc gearboxes at 50 hours of operation to catch any manufacturing fill inconsistencies, then annually thereafter. Use only the oil type specified for the individual disc gearboxes, which may differ from the main gearbox specification on some models.<\/div>\n<\/details>\n
    \nDoes mowing speed affect disc gearbox wear rate?+<\/span><\/summary>\n
    Ground speed (forward mowing speed) affects disc gearbox load through crop throughput rate. A disc mower operating at 6 mph is processing approximately 50% more crop per unit time than at 4 mph \u2014 the blades encounter more cutting resistance per revolution, and the crop fed through the disc bar cross-section is denser. This higher throughput rate increases the average torque load on the individual disc gearbox bevel gear sets, which accelerates bevel tooth wear and bearing fatigue. Operations that routinely mow at maximum speed in heavy first-cut alfalfa should anticipate 20\u201330% shorter individual gearbox service life compared to moderate-speed mowing in lighter crops. The relationship between mowing speed, cut quality, and equipment loading is analyzed in detail in the mowing and conditioning guide.<\/div>\n<\/details>\n<\/div>\n<\/div>\n

    <\/p>\n

    \"foragebaler.com<\/p>\n

    Get Gearbox Oil Specs and Maintenance Documentation for Your Mower<\/h3>\n

    Oil specifications, blade bolt torque values, and slip clutch adjustment gaps documented with every mower before delivery. Our team supports maintenance questions through the machine’s full service life.<\/p>\n

    Get Mower Specifications<\/a><\/p>\n<\/div>\n

    Editor: Cxm<\/p>\n<\/div>","protected":false},"excerpt":{"rendered":"

    Mower Mechanical Maintenance Guide Disc Mower Gearbox Maintenance: Oil, Slip Clutch, and Blade Bolts A disc mower’s cutting system spins individual blade discs at 3,000 RPM or higher \u2014 blade tip speeds approaching 150 mph in the most aggressive configurations. At those speeds, a missed oil change in a disc gearbox or a loose blade […]<\/p>","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_et_pb_use_builder":"","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"categories":[28],"tags":[],"class_list":["post-908","post","type-post","status-publish","format-standard","hentry","category-forage-baler"],"_links":{"self":[{"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/posts\/908","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/comments?post=908"}],"version-history":[{"count":2,"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/posts\/908\/revisions"}],"predecessor-version":[{"id":910,"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/posts\/908\/revisions\/910"}],"wp:attachment":[{"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/media?parent=908"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/categories?post=908"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/foragebaler.com\/id\/wp-json\/wp\/v2\/tags?post=908"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}