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§ 16

Spectacles and wheel lift: crane truck's axle load

How much weight is left on the crane vehicle's front axle when the car hangs from the tow ball?

Answer: When you lift a car in the frame (wheel lift), the load hangs behind the crane vehicle's rear axle. The load presses the rear axle down and at the same time lifts the front axle, like a seesaw. If the front axle gets too light, the crane vehicle steers and brakes poorly. The calculator works out the crane vehicle's axle load with the load on the frame from the weighed axle loads without load, axle distance and distance from rear axle to lift point.

Formula

Moment equilibrium about the rear axle: the front axle is relieved by P × e ÷ L

The rear axle gets P × (L + e) ÷ L, so more than the load itself

P = last på brillen, e = afstand fra bagaksel til løftepunkt, L = akselafstand

Forakseltryk med last = forakseltryk uden last − P × e ÷ L

Symbols and units in the formula
SymbolMeaningUnit
foraksel_tomCrane vehicle front axle load without load (weighed)kg
bagaksel_tomCrane vehicle rear axle load without load (weighed)kg
akselafstandCrane vehicle axle spacingm
last_brilleLoad on tow ball (axle load on lifted axle)kg
udhaengDistance from rear axle to lift pointm
=Front axle load with loadkg
=Rear axle load with loadkg
=Front axle's share of total weight%
=Relief of the front axlekg

Validity and source

Source
Statics (moment equilibrium about rear axle)

Does not apply when

  • Use the crane truck's weighed axle load and the allowed axle loads in the registration certificate. How large the front axle load must be at least is stated in the crane truck's and lift's instruction book. The calculator only calculates the moment itself.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

  • Crane vehicle front axle load without load (weighed)3,200 kg
  • Crane vehicle rear axle load without load (weighed)2,800 kg
  • Crane vehicle axle spacing4.20 m
  • Load on tow ball (axle load on lifted axle)900 kg
  • Distance from rear axle to lift point2 m

Giver

  • Front axle load with load2,771 kg
  • Rear axle load with load4,129 kg
  • Front axle's share of total weight40.2 %
  • Relief of the front axle429 kg

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Sådan bruger du den

Når du løfter en bil i brillen, hænger den bag kranvognens bagaksel. Den trykker bagakslen ned og letter forakslen. Bliver forakslen for let, styrer og bremser kranvognen dårligt. Beregneren regner akseltrykket med bilen i brillen, så du kan se, om du skal løfte den anden ende, flytte løftepunktet eller køre bilen op på ladet i stedet.

Step by step

  1. Vej kranvognens akseltryk uden last, eller find dem i vejesedlen.
  2. Find akseltrykket på den aksel, du løfter. Det er lasten på brillen.
  3. Mål afstanden fra kranvognens bagaksel til brillens løftepunkt.
  4. Aflæs akseltrykkene og forakslens andel, og sammenlign med de tilladte akseltryk og producentens krav til forakseltryk.

Typical errors

  • At bruge hele bilens vægt som last på brillen. Det er kun akseltrykket på den løftede aksel.
  • At glemme, at bagakslen får MERE end lasten, fordi den også tager det, forakslen mister.
  • At skyde brillen langt ud uden at regne igen. Hver meter ekstra udhæng letter forakslen mere.

Examples from everyday life

Personbil løftet i forenden

Du løfter en forhjulstrukket bil i forenden. Forakslen vejer ca. 900 kg. Kranvognen har 3.200 kg på forakslen og 2.800 kg på bagakslen, 4,2 m akselafstand og 2,0 m fra bagaksel til løftepunkt.

Crane vehicle front axle load without load (weighed)
3,200 kg
Crane vehicle rear axle load without load (weighed)
2,800 kg
Crane vehicle axle spacing
4.2 m
Load on tow ball (axle load on lifted axle)
900 kg
Distance from rear axle to lift point
2 m

It means: Forakslen lettes med 429 kg og har nu 2,771 kg. Bagakslen får 4,129 kg, og forakslen bærer 40.2 % af den samlede vægt.

Tung varebil løftet bagfra

Du skal løfte en lastet varebil bagfra, hvor bagakslen vejer 2.000 kg. Brillen er skudt helt ud til 2,6 m.

Crane vehicle front axle load without load (weighed)
3,200 kg
Crane vehicle rear axle load without load (weighed)
2,800 kg
Crane vehicle axle spacing
4.2 m
Load on tow ball (axle load on lifted axle)
2,000 kg
Distance from rear axle to lift point
2.6 m

It means: Forakslen mister 1,238 kg og har kun 1,962 kg tilbage, mens bagakslen får 6,038 kg. Med 24.5 % på forakslen skal du tjekke kranvognens grænser, skyde brillen ind eller køre varebilen op på ladet.

Rule of thumb

Forakslen lettes med lasten gange udhænget delt med akselafstanden. Ved 2 m udhæng og 4 m akselafstand mister den halvdelen af lasten.

How to check the answer

For- og bagakseltryk lagt sammen skal give kranvognens vægt plus lasten på brillen.