GB2103574A - Booms for cranes or the like - Google Patents

Abstract

A crane or the like (e.g. an access platform or excavator), has a boom 10 lifted or lowered by one or more hydraulic rams or the like 16, the or each ram acting on a cradle 13 on which the boom 10 rests and in which it is free to move, the cradle 13 being linked 19, 20 to the platform 30 or the base of the crane or the like, at a point between the foot of the ram(s) 16 and the foot of the boom or at the foot of the boom 12. For clarity the cradle 13 is shown spaced from the boom 10. A holdfast 14, shown separated from the cradle 13, holds the boom 10 in the cradle 13 whilst allowing it to slide freely longitudinally in the cradle 13. A hook 22 may be provided on the cradle 13 to enable the crane to lift light loads when the boom 10 is dismounted. In a further embodiment the cradle may be lowered out of contact with the boom during transport. <IMAGE>

Description

SPECIFICATION Improvements in and relating to booms for cranes or the like The present invention relates to cranes having hydraulically operated derricking booms.

In such cranes one or more hydraulic rams are normally positioned underneath the boom and are positively connected between the platform or base from which the crane boom pivots and the crane itself. The crane boom has therefore two points of attachment to the platform or base, one point being the ram attachment point and the other being the boom pivot point. During the manufacture of such cranes it is essential if the boom is to work at low angles as is the case in a mobile crane that the machining of the boom pivot holes relative to the derrick ram pivot holes or vice versa is carried out with a very tight tolerance. This is because at low elevations the angle between the derrick rams and boom is very small and very small differences in hole centres can cause large changes in the limiting angles of the boom.Maintaining the tight tolerance required is a costly and time consuming process.

A further problem is that stresses produced in the boom in the area about the ram attachment point are high and fluctuate from tension to compression due to high locally applied loads and moments. Whilst allowance for these stresses presents no particular problem, nevertheless when duty cycle work fatigue are design parameters, the penalties are significant. For example where the stress is an item changes from tension to compression the permissable working load may be only about 60% of that where the stress remains in tension or compression for any given number of working cycles. Similar disadvantages can also occur when the stress changes too rapidly even when only in tension or only in compression.

A crane or the like apparatus according to the invention comprises a boom lifted or lowered by one or more hydraulic rams, the or each ram acting on a cradle on which the boom rests and in which it is free to move, the boom being pivotally mounted at its foot, the cradle being linked to the platform or base of the crane.

By freely cradling the boom at its lifting point the boom stresses are evened out and the expensive gauging of the pivot to lifting points is eliminated.

A further advantage also results in that when the crane in accordance with the invention is a mobile crane and the boom is so long that a trailer needs to be provided to support the boom head, the derrick lifting ram(s) can be lowered so that the cradle is out of contact with the boom which then ensures that the boom is only supported at its head and foot and thus avoids undue stress and undue wear on hydraulic seals which would otherwise be caused at the derrick lifting point if this was fixed as in known cranes.

Preferably the pivoting of the boom to the crane base or platform is by means of a slot and pin arrangement. Suitably the boom foot is slotted. This enables the mounting of the boom to the crane base to be much simpler and is also advantageous if the boom has to be removed from or replaced on the crane.

In order to measure the load on the boom the cradle can be provided with a compression load cell. Because of the free cradling of the boom, such a load cell will indicate a true boom loading without interference from indeterminate stray parameters caused by friction, stress and so on.

A tension link or links may be pivotally connected between the cradle and platform with the link pivots to the base a platform being located nearer the foot of the boom than the position of the ram pivot to the base and preferably at the boom foot pivot position. When two links are used a derrick arrangement is created by the ram and the two links.

In order to prevent the boom bouncing in the cradle, a holdfast comprising a simple member extending from the cradle and over the boom is preferably provided.

Since the cradling arrangement of the invention remains a lifting arrangement even with the boom removed, the crane without a boom, can be used to lift light loads.

Embodiments of the invention will now be described by way of example with reference to the drawings in which: Figure 1 shows a mobile crane in which the derrick ram is fixed in a known manner to the boom, the figure including an associated boom bending moment diagram.

Figure 2 is a perspective view showing a mobile crane according to the invention with the boom lifted out of its cradle to show this more clearly, Figure 3 is a perspective view showing the mobile crane of Fig. 2 with the boom partially withdrawn from its foot pivot to show the foot pivoting arrangements more clearly, Figure 4 is a diagrammatic side elevation of the cradling arrangements of the crane of Fig.

2 with an associated boom bending moment diagram, and Figure 5 is a side elevation of a further mobile crane according to the invention with the boom supported on a trailer.

In Fig. 1 which shows a known type of crane the boom 1 is pivoted at 2 to the rotatable crane platform 3 and the boom is derricked up or down by means of a ram 4 pivotally fixed to the boom at 5 and to the crane platform at 6. The boom bending moment is diagrammatically shown at 7 where it can be seen that the bending moment sud denly changes at point 8. As will be later shown and further described with reference to Fig. 4, the bending moment of cranes according to the present invention does not have this sudden change.

In Figs. 2 and 3 there is shown a crane according to the invention in which a boom 10 having a slotted or forked foot mounting 11, is pivotally mounted on a pin 1 2 to the rotatable platform. The boom rests freely in a cradle 1 3 (Fig. 2 shows the boom lifted out of the cradle 1 3) and a holdfast 1 4 (shown removed in Fig. 2) is fixed to the cradle suitably by pins or bolts at 1 5 and 15' so as to hold the boom in the cradle whilst allowing it freely to slide longitudinally on the cradle.

The cradle 1 3 is pivotally mounted to a derricking ram 1 6 at point 17, the ram 1 6 being mounted pivotally to the crane platform 30 at 1 8 (see Fig. 3). Two tension links 19 and 20 extend from pivot point 1 7 to the boom foot pivot pin 1 2.

A hook 22 may be provided on the cradle at 1 7 to enable the crane to lift light loads when the boom is dismounted, the ram and tension links forming a derricking mechanism.

In Fig. 4 a boom bending moment diagram 25 for the crane of Fig. 2 shows that no sudden change in the diagram occurs unlike that occurring in the known type of crane shown in Fig. 1.

In order to monitor boom loads a compression load cell may be provided on the cradle 13 at 28.

In Fig. 5 a large telescoping boom mobile crane 35 according to the invention is shown which is provided with a trailer 36. The crane has a boom cradle 37 connected to a ram 38 and tension links 39, the boom 40 being pivoted at 41 to the platform 42. The ram 38 (or there can be two rams) is pivoted to the platform 42 at 43 and to the cradle at 44. In the transporting position shown in the figure the trailer has a boom support 46 which supports the boom 40 and the cradle 37 is lowered out of cradling contact with the boom so that the boom load is supported between the boom foot pivot 41 and support 46 only.

This arrangement greatly reduces the wear and tear on the derricking ram or rams.

Whilst the above description is particularly relevant to cranes, apparatus in accordance with the invention may comprise access platforms, that is, aerial lifts, excavators or any other boom equipped apparatus. Hence the term "or the like" is applied to indicate such boom equipped apparatus.

Claims (8)

1. A crane or like apparatus comprising a boom lifted or lowered by one or more hydraulic rams or the like, the or each ram acting on a cradle on which the boom rests and in which it is free to move, the boom being pivotally mounted at its foot to the platform or base of the crane or the like and the cradle being linked to the platform.

2. A crane or the like as claimed in Claim 1 in which the foot of the boom is pivoted to the base or platform by means of a slot and pin arrangement, the slot being located in either the foot of the boom or the base, and a pin provided on the base or the foot of the boom respectively.

3. A crane or the like as claimed in Claim 1 or Claim 2 in which the cradle is connected to the base or platform through a tension link or links pivotally connected both to the cradle and to the platform at a point on the boom pivot side of the ram pivot so as to constitute a derricking arrangement.

4. A crane or the like as claimed in Claim 3, in which the tension link or links is pivotally connected to the platform or base at the boom pivot position.

5. A crane or the like as claimed in any one of the preceding claims in which the boom is held loosely in the cradle by a holdfast.

6. A crane or the like as claimed in any of the preceding claims in which the cradle is provided with a hook so that the crane or the like can be used to lift relatively light loads without a boom.

7. A crane or the like as claimed in any one of the preceding claims being in the form of a mobile crane in which the boom when lowered has its front end supported by an extension of the crane structure or by a separate trailer, so that the ram can be lowered to remove the cradle from contact with the boom, during travel.

8. A mobile crane substantially as hereinbefore described with reference to Figs. 2 to 4 or 5 of the accompanying drawings.