Jacking, skidding and rolling: friction, push forces, support reactions and cribbing

Where cranes cannot reach or are not economic, heavy items such as transformers, vessels and modules are moved by jacking, skidding on tracks, or rolling on skates and rollers. The forces are much smaller than the weight, which is the point, but the calculation is less forgiving than it looks: friction is uncertain, slopes matter, and support reactions on a stiff load can be very uneven.

1. Friction and the push or pull force

To move a load horizontally the push or pull must overcome friction. On a slope it must also lift the load up the slope (or hold it back on the way down):

F = μ × W ± W × sin θ   (+ uphill, − downhill; for small slopes sin θ ≈ gradient)

Friction values vary widely with surface condition and lubrication. The figures below are typical starting values for planning only; use the supplier’s data for skidding systems, and allow for a higher breakout (static) value at the start of each push.

Worked example: skidding a transformer

A 120 t transformer on a skid frame is pushed along greased steel skid tracks up a gradient of 1 in 200 (0.5%). Take a breakout μ of 0.15 and a moving μ of 0.10.

• Breakout: F = 0.15 × 120 + 120 × 0.005 = 18.0 + 0.6 = 18.6 t.
• Moving: F = 0.10 × 120 + 0.6 = 12.6 t.
• Two 15 t push-pull units give 30 t, about 62% utilised at breakout, leaving margin for a dirty track or a higher friction than assumed.
• The push units’ anchorage (gripper on the track, or lugs) must take the same force, and the skid track must be tied together or anchored so it is not pushed along with the load.

Downhill and rolling loads. On skates with μ = 0.02 and a 3% slope, the slope component (0.03 W) exceeds the rolling resistance (0.02 W) by 0.01 × 120 = 1.2 t: the load will run away unless it is held back. Always provide a hold-back (a winch, a jack or a brake) when the gradient may exceed the rolling resistance, and stop blocks at the end of the run.

2. Support reactions on skates: make it determinate

A stiff load on four or more supports is statically indeterminate, like a four-leg bridle (Chapter 3). A small difference in track level can leave one skate nearly unloaded and another overloaded. The standard answer is to group the supports into three points: link two or more skates hydraulically (so they share pressure equally) or with a rocking beam, so that each group acts as one point.

Example. A 120 t module, 8.0 m between front and rear supports. The two front skates are linked to act as point 1 on the centreline; the rear skates are separate points 2 and 3, 3.0 m apart. The CoG is 4.5 m from the front and 0.3 m towards point 3.

• Front (point 1): 120 × (8.0 − 4.5) ÷ 8.0 = 52.5 t, or 26.25 t per linked skate.
• Rear total: 67.5 t. Point 3: 67.5 × (1.5 + 0.3) ÷ 3.0 = 40.5 t; point 2: 27.0 t.
• With 50 t skates, point 3 is at 81%. If the CoG tolerance is ±0.3 m sideways, point 3 could reach 47.3 t (95%). Use larger skates there, or link the rear skates instead and check the front pair.

3. Jacking

Jack force from pressure

A hydraulic jack’s force is the pressure times its effective piston area: F = p × A. A jack rated 100 t at 700 bar (70 MPa) has an effective area of about 981 kN ÷ 70 MPa ≈ 14,000 mm² (140 cm²). With the gauge reading 450 bar (45 MPa): F = 45 × 14,014 = 631 kN = 64.3 t. Reading the pressure at each jack is a good way to check the weight and CoG of a load (Chapter 1), provided the gauges are calibrated and friction in the jacks is allowed for.

Jacking one end

Lifting one end of a load with the other end on its support works like the tilting check in Chapter 1. The jacks at the lifted end carry W × x ÷ L, where x is the horizontal distance from the far support to the CoG. Example: an 80 t vessel on saddles 6.0 m apart, CoG 2.6 m from saddle A, jacked at saddle B: jack load = 80 × 2.6 ÷ 6.0 = 34.7 t. Two 30 t jacks share it at about 58% each, with margin for uneven sharing. As the end rises, the load tilts and wants to slide towards the low end: restrain it, keep jack heads level and use swivel or tilt saddles.

Rules for jacking

• Use jacks within their rated load and stroke, on firm, level bearing, with the load centred on the jack head.
• Follow the load down with packing (“chase the load”) as it rises, so that it is never supported on hydraulics alone. Use mechanical lock nuts where the jacks have them.
• Lift in small, equal steps. Watch the level of the load and the pressures at every step.
• Provide lateral restraint against the load sliding off the jacks. Many procedures design restraint for a horizontal force of a few percent of the vertical load.
• Never put any part of your body under a load supported only by jacks.

4. Cribbing

Cribbing (stacks of timber in alternating layers) supports loads during jacking and as temporary stools. Check three things:

1. Bearing on the timber. Timber is weak across the grain. A 300 mm diameter jack base carrying 64.3 t gives 631 kN ÷ 70,700 mm² = 8.9 MPa, well above what hardwood can take across the grain (typically a few MPa allowable). A 600 × 600 mm steel spreader plate reduces it to 631 kN ÷ 360,000 mm² = 1.75 MPa.
2. Bearing on the ground under the bottom layer, as for crane mats (Chapter 8).
3. Stability. Keep stacks low and wide. A common rule of thumb limits the stack height to about one and a half to two times its base width; follow your procedure. Load the stack at its centre, use sound timber of the same size in each layer, and never mix soft and hardwood in one stack.

4a. Hydraulic gantries

A hydraulic gantry is a set of telescopic lifting towers on tracks, joined by a header beam, used for lifting and moving heavy loads in buildings and plants. Its capacity falls as the towers extend through their stages, and the maker’s chart gives the capacity for each stage and for each header beam span. Three points need particular care:

• Side loads. The towers are slender columns. Charts give limited allowances for side load from out-of-level track, wind, travel acceleration and load swing. Track must be level within the maker’s tolerance.
• Synchronisation. The legs must lift and travel together; the control system’s synchronisation and the allowed difference in leg height are part of the rating.
• Load sharing. With four legs and a stiff header arrangement, the shares are indeterminate in the same way as four skates. Use the maker’s guidance on CoG position and monitor leg pressures.

5. Common mistakes

• Using the moving friction value at breakout, or a value for a clean greased track on a dusty one.
• Ignoring a slight down slope on rollers, so the load moves on its own.
• Supporting a stiff load on four independent skates and assuming a quarter each.
• Leaving the load on jacks without packing while people work nearby.
• Putting a jack directly on timber or soft ground without a spreader.
• Pushing against an anchor or structure that has not been checked for the push force.

6. Field checklist

• Weight and CoG known; support reactions calculated for three-point groups with the CoG tolerance.
• Push or pull force calculated for breakout and moving, including any slope; equipment at no more than the planned utilisation.
• Hold-back available where the gradient can exceed rolling resistance; end stops fitted.
• Track level, aligned, clean and lubricated as specified; joints and anchors secure.
• Jack gauges calibrated; packing ready at every jacking point; lateral restraint in place.
• Cribbing and spreader plates sized for bearing; stacks stable.

7. Self-check questions

General guidance only. It does not replace the manufacturer's data, the applicable standards or the judgement of a competent person.

References

Technical references consulted for this chapter. The text and worked examples are original to LiftPlan Pro.

• Rigging Handbook, 2nd edition (Bechtel Equipment Operations, 2002).
• ASME B30.1 Jacks, Industrial Rollers, Air Casters and Hydraulic Gantries.
• J. Keith Anderson, Rigging Engineering Calculations (Industrial Training International).

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