A Post Crane is a small fixed-mast jib crane mounted to a vertical post — either a permanent ground-anchored pole or a vehicle-mounted column — that swings a boom around the post to lift, place or pull loads. Typical capacity runs 500 to 4,000 lbs at 6 to 14 ft of reach with manual or hydraulic boom rotation. It exists to set utility poles, fence posts, well casings and tower sections without bringing in a full mobile crane. You see them on Altec line trucks and on farm-built post-setters across rural North America.
Post Crane Interactive Calculator
Vary load, boom reach, and optional boom self-weight to see the mast-base overturning moment update in the crane diagram.
Equation Used
The post crane base must resist the overturning moment from the suspended load plus any boom self-weight. Load moment is load times horizontal reach; boom moment is boom weight times its center-of-gravity distance from the mast.
- Static vertical lift with no shock or wind loading.
- Reach is the horizontal distance from mast centerline to load.
- Boom center of gravity is measured horizontally from the mast.
- Pounds are treated as pounds-force.
Inside the Post Crane
A Post Crane works on the same principle as any jib crane — a vertical mast carries the load through a thrust bearing at the top and a radial bearing at the bottom, and a horizontal or angled boom pivots around that mast to swing the load through an arc. The mast itself is the structural backbone. On a truck-mounted unit like an Altec DM47 derrick digger the mast is a 6 to 8 inch schedule 80 pipe bolted to a sub-frame; on a yard-built post setter it might be a 4 inch square HSS section sleeved into a concrete-anchored ground socket. The boom hangs off the mast through a slewing collar — usually two tapered roller bearings or a single turntable bearing — and a winch line or hydraulic cylinder picks the load.
Why build it around a single post instead of a wider footprint? Because the whole point of this mechanism is to work in tight corridors where a mobile crane's outriggers won't fit. A guyed mast crane on a pole-line job sits in the right-of-way and lifts 40 ft Class 2 wood poles straight off the trailer with nothing more than the truck wheels for ballast. A swing arm derrick on a fence-post setter swings out 8 ft, drops a 1,000 lb concrete corner post into a freshly augered hole, and swings back without the operator ever leaving the seat.
Tolerances matter more than people think. The slewing bearing must be preloaded to roughly 0.001 to 0.003 inch axial play — too loose and the boom tip wanders 2 to 3 inches under load, which makes setting a pole into a 12 inch dug hole frustrating. Too tight and the bearing draws 30 to 40 percent more torque to swing, which kills the operator's arms on a manual unit. Common failure modes are mast-base weld cracking from cyclic side loading, slewing bearing brinelling from shock-loading a stuck post, and winch wire-rope birdcaging from side pulls. If you see the boom drooping under no load, your top thrust bearing has lost preload — pull it apart and shim before the races spall.
Key Components
- Mast: The vertical structural column the entire crane pivots around. Typically 4 to 8 inch schedule 80 pipe or square HSS, length 8 to 16 ft above the mounting plane. Must be plumb within ¼° or the boom will self-rotate downhill under gravity.
- Slewing Bearing: The rotational joint between mast and boom. On light units a pair of tapered roller bearings; on heavier units a single 4-point contact turntable bearing 12 to 24 inch bore. Axial preload tolerance is 0.001 to 0.003 inch.
- Boom: The horizontal or angled arm that carries the load away from the mast. Typically 6 to 14 ft long, fabricated from rectangular HSS or a fishbelly truss. Boom tip deflection under rated load should stay under 1 percent of length.
- Winch and Wire Rope: Provides the vertical lift. Manual hand winch on small post-setters (1,000 lbs working load with a 4:1 drum), hydraulic planetary winch on truck-mounted units (3,000 to 8,000 lbs line pull). Wire rope is usually 6×19 IWRC at 5/16 to 1/2 inch diameter.
- Base Mount and Anchor: Either a bolted flange on a truck sub-frame, a four-bolt baseplate set into a concrete pier, or a guyed-mast plate with three guy-line eyes at 120°. The anchor must resist the overturning moment — for a 4,000 lb load at 10 ft reach that's 40,000 ft-lbs.
- Boom Rotation Drive: Manual handle on small units, hydraulic rotary actuator (typically Eaton or Helac vane motors at 5,000 to 10,000 ft-lbs) on truck-mounted derricks. Rotation speed is intentionally slow — 1 to 3 RPM — so loads don't pendulum.
Where the Post Crane Is Used
Post Cranes show up wherever you need to lift a heavy vertical object into a hole, or pull one out, in a place a full mobile crane cannot park. They cover everything from utility line work to ranch fencing to wellhead servicing, and the same basic kinematics scale from a 500 lb hand-winch unit to a 6-ton truck-mounted derrick. The decision to use one usually comes down to access, not capacity — if you can fit a mobile crane you almost always will, but on a 12 ft easement between two cornfields a post crane is the only thing that gets the job done.
- Electrical Utility: Altec DM47 and DM4250 digger derricks setting Class 2 and Class 3 wood distribution poles for Hydro One line crews across rural Ontario.
- Agriculture and Ranching: Worksaver HPD-22 and Shaver HD-10 hydraulic post drivers and post-setters on John Deere 5075E tractors, used for installing fence corner posts and gate posts on cattle operations.
- Telecommunications: Guyed mast cranes used to lift Rohn 25G and 45G tower sections during self-supporting tower assembly for rural cellular sites.
- Oil and Gas Well Servicing: Single-mast pulling units like the Cooper LTO-450 on stripper wells in the Permian Basin, pulling sucker rod strings up to 6,000 ft.
- Marine and Dock: Fixed pier-mounted davit cranes at small commercial fishing wharves on the BC coast, lifting traps, motors and small skiffs up to 2,000 lbs.
- Construction and Heritage Restoration: Yard-built guyed stiffleg derricks used by stone masons to set granite gateposts and 3-ton dimensional stone blocks on heritage church restorations.
The Formula Behind the Post Crane
The number that decides whether a Post Crane is sized correctly is the overturning moment at the base. Boom reach times load gives you the moment the mast tries to tip about its base, and that moment has to stay below what the anchor or counterweight can resist. At the low end of typical reach — say 4 ft — even a 4,000 lb load only generates 16,000 ft-lbs, which a bolted truck sub-frame handles easily. At the high end — 14 ft of reach — the same load generates 56,000 ft-lbs and now you need outriggers, guy lines or a much heavier counterweight. The sweet spot for most truck-mounted units sits around 8 to 10 ft of reach, where moment, working radius and operator visibility all balance.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Mtip | Overturning moment at the mast base | N·m | ft-lbs |
| Wload | Weight of the suspended load at the boom tip | N | lbs |
| Rboom | Horizontal reach from mast centreline to load hook | m | ft |
| Wboom | Self-weight of the boom assembly | N | lbs |
| Rcg | Distance from mast centreline to boom centre of gravity | m | ft |
Worked Example: Post Crane in a municipal cemetery monument crane
A monument company in Saint John New Brunswick is specifying a yard-mounted Post Crane to set granite headstones and 1,800 lb upright monuments onto concrete bases inside an existing cemetery where row spacing prevents a mobile crane from entering. The mast will be a 6 inch schedule 80 pipe set in a 4 ft deep concrete pier. The boom is 10 ft long and weighs 220 lbs with the centre of gravity at 5 ft from the mast.
Given
- Wload = 1800 lbs
- Rboom = 10 ft (nominal reach)
- Wboom = 220 lbs
- Rcg = 5 ft
Solution
Step 1 — at nominal 10 ft reach, compute the load moment about the mast base:
Step 2 — add the boom self-weight moment, which is small but not negligible:
Step 3 — total overturning moment at nominal reach:
At the low end of the typical operating range — pulling the load in to 6 ft reach to land it on the base — the moment drops to Mtip,low = 1800 × 6 + 220 × 3 = 11,460 ft-lbs. The crane feels stable, the operator can swing fast, and the concrete pier sees less than 60 percent of its design moment. At the high end, picking a stone off a delivery pallet at 14 ft reach, the moment climbs to Mtip,high = 1800 × 14 + 220 × 7 = 26,740 ft-lbs. That's a 40 percent jump from nominal and it's where most yard-built post cranes get into trouble — the pier rebar starts to pull, the mast starts to lean visibly, and the boom tip droops.
The 4 ft deep concrete pier needs to resist that worst-case 26,740 ft-lbs with a safety factor of 2, so design for at least 53,500 ft-lbs of overturning resistance. A 30 inch diameter pier 4 ft deep in firm till gives you roughly that much, no more.
Result
Nominal overturning moment at 10 ft reach is 19,100 ft-lbs. That's the load the mast base, the slewing bearing and the concrete pier all have to handle every cycle, and on a yard crane it means a 30 inch diameter pier socketed at least 4 ft into firm soil. Pulled in to 6 ft the moment drops to 11,460 ft-lbs and the crane feels rock solid; pushed out to 14 ft it climbs to 26,740 ft-lbs and you're flirting with pier rotation if the soil is anything softer than dense till. If your measured mast lean exceeds ½° under load when the calculation says it should be plumb, the most likely causes are: (1) the pier concrete cured below 25 MPa and is crushing locally at the baseplate edge, (2) the four anchor bolts were set in oversized sleeves and are slipping under tension rather than bearing on the concrete, or (3) the soil around the pier is saturated and the passive resistance has dropped 50 percent from the dry design value.
Choosing the Post Crane: Pros and Cons
A Post Crane is one of three common ways to lift a heavy vertical object into a tight space. The other two are a small mobile crane (boom truck or carry deck) and a tracked mini-crane like a Maeda MC305. Each one wins on a different axis — moment capacity, footprint, setup time, and cost — and the decision usually comes down to which constraint is binding on your specific job site.
| Property | Post Crane | Boom Truck (Mobile Crane) | Tracked Mini-Crane (Maeda-style) |
|---|---|---|---|
| Typical load capacity | 500 to 8,000 lbs | 8 to 50 tons | 1 to 6 tons |
| Working radius | 4 to 14 ft | 20 to 100+ ft | 10 to 60 ft |
| Footprint with outriggers / anchors deployed | 3 to 6 ft (just the mast and pier) | 20 to 25 ft square | 8 to 12 ft square |
| Setup time on site | 0 minutes if permanent, 30 min if guyed | 10 to 20 minutes | 5 to 10 minutes |
| Capital cost (typical 2024 used market) | $3,000 to $40,000 | $80,000 to $400,000 | $120,000 to $300,000 |
| Slew speed | 1 to 3 RPM (manual or hydraulic) | 1 to 2 RPM | 0.5 to 1.5 RPM |
| Best application fit | Repetitive picks in tight corridors — pole lines, fence rows, cemeteries | General lifting where access allows | Indoor lifts and confined-access steel erection |
Frequently Asked Questions About Post Crane
The most common cause is sub-frame flex, not outrigger settlement. The mast bolts to a sub-frame welded to the truck chassis, and at full reach the moment twists the chassis rails themselves. On a single-rear-axle Class 5 truck you can see 1 to 2 inches of cab-side lift at 4,000 lbs and 12 ft reach because the chassis rails are only 8 inch deep C-channel.
Check it by laying a digital level on the mast flange before and during the lift. If you see more than 0.5° change with outriggers loaded, you need either a heavier truck (tandem axle with 10 inch rails) or a torsion box welded between the sub-frame and the chassis. This is why utility line trucks like the Altec DM47 are always built on Freightliner M2 or International 7400 chassis, never on a half-ton.
The deciding number is your worst-case overturning moment divided by the available footprint. A free-standing pier needs to resist the full moment through soil bearing and rebar tension — for 25,000 ft-lbs that's typically a 30 inch diameter pier 4 to 5 ft deep, which costs $1,500 to $3,000 in concrete and excavation. A guyed mast splits the moment into three guy-line tensions at 120° spacing, so each guy only sees about one-third the load and the base just needs to resist vertical compression.
Go guyed if you have 15 ft of clear radius around the mast for the guy anchors. Go pier if you don't — guy lines in a working yard get cut by forklifts within a year. For most monument shops and small fab yards, a pier wins on practicality even though it's more expensive up front.
That's almost always brinelling from shock loading, not normal wear. Brinelling is permanent indentation of the bearing race by the rolling elements when a static or impact load exceeds the race's yield strength at the contact point. On a post crane it happens when you try to free a stuck pole — the load suddenly releases, the boom whips upward, and the slewing bearing takes a vertical impact spike of 3 to 5 times rated load.
You'll feel it as evenly-spaced rough spots at intervals matching the ball or roller pitch. Once it's brinelled there's no fix — replace the bearing and add a shock-absorbing snubber or a torque-limiting clutch on the winch line so a sudden release can't slam the boom up. A simple inline rubber bumper on the boom stop saves a $1,200 turntable bearing.
If the calculation only accounted for boom bending you missed two larger contributors. Mast bending below the slewing bearing typically adds 1 to 2 inches at the boom tip on a 12 ft mast, because the mast is a cantilever beam too. And the slewing bearing itself has internal clearance — even a properly preloaded turntable bearing tilts 0.05 to 0.10° under moment load, which translates to another 1 to 2 inches at a 10 ft reach.
Add all three sources — boom bending, mast bending, and bearing tilt — and 4 inches of total tip droop on a 4,000 lb load at 10 ft is exactly what theory predicts. The fix isn't more boom stiffness; it's a larger turntable bearing and a heavier wall mast. Going from 6 inch sch 80 to 8 inch sch 80 mast cuts mast deflection by about 60 percent.
You can, but the load case is completely different and most rated lift capacities don't apply. Pulling a stuck post is a vertical line pull with potentially huge transient loads when the post breaks free from the soil. Static pull-out force on a 4 ft embedded pole in clay can exceed 8,000 lbs, and at the moment of release the line goes slack and the boom recoils.
Use a winch with a pull rating at least 1.5× the expected breakout force, keep the boom radius short (4 to 6 ft) so the moment stays manageable, and never pull at an angle — side loading the boom will fold it. Many dedicated post pullers like the Worksaver HPP series use a clamping jaw on a vertical slide rather than a swinging boom for exactly this reason.
Pendulum period depends on the rope length from the boom tip to the load, not the load weight. For a 6 ft drop the natural period is about 2.7 seconds, so the load wants to swing back and forth every 2.7 seconds. If you stop the boom rotation faster than about half that period the load keeps moving and overshoots — typically 6 to 12 inches of pendulum at the hook.
Rule of thumb: keep peripheral boom-tip speed under 1 ft/s during the last 30° of swing before a stop. On a 10 ft boom that means slewing no faster than about 1 RPM through the final approach. Hydraulic post cranes use proportional valves with deceleration ramps for this reason; on manual units you just have to feather the handle.
References & Further Reading
- Wikipedia contributors. Crane (machine). Wikipedia
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