MS 416 P t k tMS 416 Pesawat angkat

LOAD AND FORCELOAD AND FORCE

Tito Shantika, MengMechanical Engineering Department

Classification of LoadsClassification of Loads

Loads that affect cranes can be organized into three categoriesthree categories: regular, occasional, and exceptional exceptional.

Including static and dynamic load.

Regular loadsRegular loads

Regular loads derive from gravity effects and from acceleration ind ced b the crane dri es andacceleration induced by the crane drives and brakes. These are the loads that repeatedly occur during normal crane operation.

The occasional loadThe occasional load

The occasional load category includes in-service wind snow ice and temperat re effects k i iwind, snow, ice, and temperature effects; skewing is added to this group for bridge-, gantry-, and portal-mounted cranes.

Wind is almost always presentwhen a crane is Wind is almost always presentwhen a crane is working, but it infrequently reaches velocities as hi h th l d f d ihigh as the value used for design.

Exceptional loadsExceptional loads

Exceptional loads are infrequent, introduced by e ents s ch as e treme storm winds erection andevents such as extreme storm winds, erection and dismantling, testing, and earthquakes.

The category of a load should not be taken as an indication of its importance.indication of its importance.

Static loadsStatic loads

Static loads associated with crane and derrick operations occ r d ring a state of rest b t alsooperations occur during a state of rest but also underlie dynamic loadings.

They derive from the lifted load, from the deadweight of the machine, and from snow or icedeadweight of the machine, and from snow or ice accumulation.

Lifted LoadsLifted Loads

The definition of lifted load is, quite simply, that which is given by the manufacturer of the particular machine in g y pquestion. The definition is not the same for all equipment.

For mobile cranes and many other types of lifting devices, it y yp g ,includes the hook block and overhaul weights as well as the live load and all accessories used to attach and hold the load.

For hammerhead tower cranes and some large-capacity lifting machines where the hoisting-system reeving is unchanging, the hook block and overhaul weights are taken

hi d d i h d f h l das machine dead weight and not as part of the load.

Dead LoadsDead Loads

Dead Loads For critical calculations, deadweights from handbooks or from the nominal dimensions offrom handbooks or from the nominal dimensions of plate stock should be adjusted to allow for normal variations.

As parts move in relation to each other during As parts move in relation to each other during machine functions, dead loads of components must b t k i th i ti iti L ffi b libe taken in their active positions. Luffing booms, live masts, and boom-hoist spreaders are common components that move with radius changes.

Effects of Load DistributionEffects of Load Distribution

As a crane superstructure swings, forces acting on its s pports redistrib te For a fi ed base machinesupports redistribute. For a fixed-base machine, compressive support reactions may change to uplift reactions that require tie-downs and vice versa.

The reactions are readily found by static equilibrium The reactions are readily found by static equilibrium with considerationof consistent strains.

Friction

Friction Earlier discussions have shown how friction is an important factor in comp ting wire rope loadsan important factor in computing wire-rope loads, lead-line forces, and overhauling weight requirements. Friction must, of course, be considered in the design of every mechanism.g y

Out of Level SupportsOut-of-Level Supports

All cranes and derricks must be mounted level to within close tolerances.

A common specification for mobile equipment permits a maximum of 1% out of level between supports. Tall and pplimber machines are more sensitive; an initial deviation from a level base will be amplified by beam-column action so that even more strict tolerances are the norm.

For machines with high CGstower cranes are the prime examplea relatively small difference in levelness of the supports can result in a significant horizontal displacement f h CG Thi l i dof the CG. This alters support reactions and component

loadings and can reduce resistance to overturning

Misalignment and SkewMisalignment and Skew

The wheels of track-mounted equipment are flanged on one or both edges or ha e lateral g ideflanged on one or both edges or have lateral guide rollers to keep the machine on the rails.

At the very least, side forces increase load on travel-train components and drive motors causingtravel train components and drive motors causing accelerated wear at the points of contact; the th l ff t i d th t t t t dthermal effect can induce the motors to cut out and interrupt travel

Earthquake

Studies of earthquake effects on cranes are few, and code de elopment in this area is in its infancand code development in this area is in its infancy. Generally, permanent installations such as bridge cranes and port cranes can be subjected to seismic analysis using the same principles as those used for y g p pother fixed structures.

earthquakeearthquake

In adopting a philosophy for earthquake resistance, the crane analyst or designer might consider one or more of h k l l lthree risk mitigation levels, or limit states.

1. The earthquake design does not cause structural damage t th All t i i th l tito the crane. All stresses remain in the elastic range.

2. The design earthquake may result in some damage that could be readily repaired and the crane restored Failurecould be readily repaired and the crane restored. Failure may occur in components that are not part of the main force-resisting system.

3. Controlled ductile yielding may result in the complete functional loss of the crane, which would be replaced, but th id f t t hi f il l th blithe avoidance of a catastrophic failure leaves the public, workers, and surroundings protected.

Dynamic LoadsDynamic Loads

In addition to the centrifugal force previously mentioned dynamic loads are for the most part thosementioned, dynamic loads are for the most part those associated with masses undergoing changes in motionmotion.

Each crane and derrick motionhoist, trolley, luff, l d l d d i f hslew, and travelproduces a dynamic force as the

motion begins and ends. Each accelerating mass within the crane is subjected

to a dynamic force following Newtons Second Law, y g ,F = ma,

Linear MotionLinear Motion

The force required to accelerate or decelerate a mass in linear motion can be fo nd simpl bmass in linear motion can be found simply by applying Newtons law if the acceleration rate is known.

For friction cranes acceleration and deceleration is For friction cranes acceleration and deceleration is not linear, as drive and braking systems are

ll bl f f t f thgenerally capable of far greater force than needed and are modulated by the operator.

Linear MotionLinear Motion

When a load is dropped in free fall, the acceleration is retarded by friction at the sheavesacceleration is retarded by friction at the sheaves and the inertia of the winch drum. Considering only gravity and friction resulting acceleration is givengravity and friction, resulting acceleration is given by

with h as the height of fall.

Taking the kinetic energy at the time of application of the brakes and adding the potential energy of the load W and dynamic rope stretch we can equate this with the potential energy in the ropesstretch , we can equate this with the potential energy in the ropes after stretching is complete. If F is the final total force in the ropes:

but the spring rate of the ropes is k = F/. Substituting this and rearranging, we get

impact forceimpact force

Although the term impact is generally, and more correctly, applied to the striking of a body by another body, it is also used to describe the increase in load effect due to dynamic causes The impact forcethe increase in load effect due to dynamic causes. The impact force Fi is then given by F W, and impact can be expressed as

Example 1Example 1

An operator allows a 5000-lb (2268-kg) load to free fall for 10 ft (3 05 m) before reali ing that it isfree-fall for 10 ft (3.05 m) before realizing that it is going too fast. In panic, he applies the brake fully, causing a virtually instantaneous stop. What peak force will develop in the rope if three parts of line p p pwere in use, friction loss can be taken as 2%, and the rope spring constant is 2300 lb/in per partthe rope spring constant is 2300 lb/in per part (402.8 N/mm per part)?

SolutionSolution

SolutionSolution

Example 2Example 2

What maximum velocity would have to be maintained to keep impact from e ceeding 30% ifmaintained to keep impact from exceeding 30% if the stopping distance is not to exceed 5 ft (1.52 m)?

SolutionSolution

Example 3Example 3

For the velocity developed in part 1, 24.88 ft/s (7 58 m/s) what stopping distance would be(7.58 m/s), what stopping distance would be needed to hold impact to 30%?

Example 4Example 4

Example 5Example 5

Wind LoadsWind Loads

At any particular location, actual wind measurements will continuously and randomly vary in both velocity and d W h h d l ddirection. Within the randomly varying pattern, a trend or average can be discerned for any time interval.

I E th t d d th d f ti i In Europe, the standard method for reporting maximum storm winds is based on a 10-min average in open country at 10 m above the ground.g

in the United States, the standard method was based on the time it takes 1 mi (1.6 km) of air to pass the monitoring point at an elevation of 10 m (33 ft). This fastest-mile wind is an average of the gusts and lulls that occur during a maximum period of 1 min when the wind is blowing at 60 mi/hr (34period of 1 min when the wind is blowing at 60 mi/hr (34 m/s).

Wind Velocity PressureWind-Velocity Pressure

Air at rest at sea level induces an ambient pressure of about 14 7 lb/in2 (101 4 kN/m2) absoluteabout 14.7 lb/in2 (101.4 kN/m2) absolute.

To put the phenomenon in scale, a change of only 1% in /normal pressure is equivalent to 21 lb/ft2 (1014

N/m2)a mighty wind capable of great destruction.

The static pressure relationship is given byThe static pressure relationship is given by (where is the density of air):

Wind Pressure on ObjectsWind Pressure on Objects

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