Análisis de la demanda

Sections 4.2 to 4.5 give the calculations of the hoisting brakes. Section 4.7 shows how the braking distance of a crane can be calculated.

Modern brake types have a brake torque which is normally calculated with a friction coefficient ofµG0,4 and they must give a brake torque of 1,6Bthe nominal motor torque up to 2,2Bthe nominal motor torque. For the braking of ‘vertical movements’, e.g. hoisting兾lowering or lowering兾hoisting the boom normally disc brakes are used.

These disc brakes have the advantage that the inertia movement of the brake disc is low, compared to that of the brake drum of a drum brake. Besides:

– One brake size may be used for different disc diameters, while a drum brake requires a different brake size for every different drum diameter.

– The friction material available for modern disc brakes allows for far higher operational speeds and temperatures of the brake lining, thus offering a high level integrity in case of emergency.

– The friction properties of non-organic disc brake friction materials are much less influenced by corrosion, pollution, and humidity.

– The life cycle of disc braking linings compared to drum brake linings – providing they are used for the same application – is 50 to 100 percent longer.

A number of specialized manufacturers sell excellent brakes. The examples shown in Tables 4.1.1 to 4.1.3 show figures from the

well-known manufacturers of brakes, Bubenzer of Kirchen in Germany.

Bubenzer provides the following figures for their disc brakes.

Table 4.1.1

J brake disc+

coupling

Brake type SB 14.11 (kgm²)

Thrustor type Ed 23兾5 Ed 30兾5

Contact load in N 2500 3400

Brake disc Brake torque MBr.Maxin Nm at an average friction ofµG0,4

250 200 270

The drum brakes are sometimes somewhat cheaper than the disc brakes; however the brake torque can be less than that of the disc brake of the same diameter.

Table 4.1.2

J brake disc+

coupling

Brake type SB 23 (kgm²)

Thrustor type Ed 50兾6 Ed 80兾6 Contact load in N 8500 12 500

Brake disc Brake torque MBr.Maxin Nm at an average friction ofµG0,4

355 935

400 1085 1600

450 1255 1850

500 1425 2100 2

560 1630 2400 3

630 1870 2750 4,5

Table 4.1.3

J brake disc+

coupling

Brake type SB 28 S (kgm²)

Thrustor type Ed 121兾6 Ed 121兾6 Ed 301兾6 Ed 301兾8 Brake disc Brake torque MBr.Maxin Nm at an average friction ofµG0,4

500 3100 5040 2

560 3550 5750 3

630 4100 6600 9700 12 500 4,5

710 4700 7600 11 200 14 400 7,5

800 5400 8800 12 800 16 500 11

900 14 700 18 900 15

1000 16 500 21 200 22

Trolley-travelling and crane-travelling mechanisms often use a plate brake. This is in fact a disc brake. These brakes are built-in in the motors on the second motorshaft. These brakes are not so easy to con-trol and adjust. The brake torque can show a tolerance of plus or minus 15 percent which makes it difficult to predict the exact brake torque which the plate brake will give.

With a crane or trolley running at a high speed, it is wise to calculate the brake distance of the crane or trolley in accordance with Section 4.7. With trolley and crane travel brakes, which can easily be adjusted, a disc brake or a drum brake can be used, released by a thrustor or a DC solenoid. The brake torque of such brakes should be taken as approximately 1,5 to 1,8 times the nominal motor torque.

With boom-hoist mechanisms it is usual to build in a disc brake or drum brake between the motor and the in-going shaft of the gearbox.

Additionally, a second, so-called emergency brake is installed on the boom-hoist drum. Should something happen during lowering the boom and if the motor speed reaches 110 percent of the nominal motor speed, a centrifugal switch activates this emergency brake.

The emergency brake used to be constructed as a bandbrake, now, caliper disc brakes are normally used. However, some crane users still prefer a bandbrake, because the brake rim under a bandbrake becomes less corroded than the brake sheave of a disc brake. To avoid rust and corrosion on the disc surface it is possible to provide the caliper disc

Fig. 4.1.1 Disc brake

Fig. 4.1.2 Drum brake

Fig. 4.1.3 Plate brake

brake with cleaning pads. Bubenzer mention 4 caliper disc brake types – see Table 4.1.4. To give a hoisting winch extra safety, in the case of a severe breakdown in the gearbox, some crane users demand an extra disc- or bandbrake on the hoist drum itself. In case of such a calamity in the gear box, these brakes must then catch the load.

Maintenance

The maintenance period of the brakes are described in Chapter 10 Maintenance. It is clear that the brakes have to be inspected, controlled, and maintained very regularly, because of the enormous problems caused if a brake should fail.

The monitoring of disc brakes

The preventative maintenance of disc brakes can be simplified by instal-ling a monitoring system. When a number of sensors are installed on

Table 4.1.4 Caliper disc brakes

Type SF10 SF15 SF24 SF40

Contact load 100 KN 150 KN 240 KN 400 KN

Friction coefficient µG0,4 µG0,4 µG0,4 µG0,4

Operating factor f G40 f G60 f G96 f G160

Operating diameter d (mm) d (mm) d (mm) d (mm) Brake torque in Nm MbrGf · d (Nm)

Fig. 4.1.4 Calliper disc brake

each brake the following items can be monitored:

– the contact pressure of each brake pad, giving an indication of the brake torque;

– the brake pad temperature;

– the brake pad wear;

– the brake disc speed;

– the position of the thrustor.

Data from these sensors are submitted and visualized via a field bus system to the main control system or via radio signals to a separate computer or modem. Bubenzer can install this CCB (Computer Con-trolled Brake) system on a series of disc brakes in one or more cranes.

Permanent monitoring of brake systems and print outs of brake reports can even be produced from the central maintenance room where further maintenance then can be organized, etc.