Selecting Steel Wire Ropes (Pemiliha/perencanaan Kawat baja) 1. Determine the ratio Dmin/d of every pulley system from the table (the values are depending on the number of bents). Diameter of the rope is expressed by the formula:

√ (9) Where: δ = diameter of one wire i = number of wires in the rope 2. The stress in a loaded rope is:

(10)

Where: σb=ultimate breaking strength of the rope (kg/cm2)/kekuatan K = Factor of safety/ faktor keamanan S = Tension of the rope (kg)/beban tali F = Coss-section area of the rope (cm2) E’ = 3/8 E (Elastic modulus --- kg/cm2)

Transforming formula (10) we obtain:

√

(11)

For ropes most frequently used in hoisting machinery are the ropes with 114,

222, and 342 wires.

For the rope with 222 wire, formula (11) becomes (E = 2.100.000 kg/cm2)

(cm2)

Maximum permissible tension in the rope is:

(12)

Where: S = maximum permissible tension (kg) P = actual breaking strength of the rope (kg) K = factor of safety Maximum working tension in the rope parts (Sw) is obtained from the formula:

Where: Q = Load being raised (kg) n = number of rope parts η = pulley efficiency (Tabel 8) η1 = rope stiffness efficiency ( 0.98)

Determination the number of bends (jumlah belokan tali):

1. Number of bends in a pulley system with one movable pulley

One bends is assumed to mean the transition of the rope from its

straight position into a bent one, or from a bent position into a straight

one. Reverse bending is count double.

2. Number of bends in a pulley system with multiple pulley

Number of bends = 3

In determining the number of bends for multiple pulleys, the compensating pulley is not considered since it remains stationery when the load is being raised or lowered.

I. Design of Rope Sheaves (Perancangan Alur Puli)

Rope sheaves (alur pulli) for steel wire ropes for various rope diameters is

illustrated in tabel 16 bellows:

Rope sheaves is illustrated in figure bellow. Figure 54 (a) Large sheaves with ribs

or holes, (b) welded rope sheaves (alur pulli yang dilas)

For compensating sheaves (Figure 55), its diameter may be taken 40 percent

smaller than the diameter of the sheaves carrying the load (diameter puli kecil

adalah 40 persen lebih kecil dari puli beban).

The ratio between the hub length and the diameter of the axle is usually taken

within the range (l/d = 1,5 to 1,8).

The unit pressure in the sheaves is:

Where: l = length of the bussing (lebar puli) D = diameter of the sheave axle (diameter poros puli) Q = load (beban) Unit pressure should not exceed the following values:

v (m/s) 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 p (kg/cm2) 75 70 66 62 60 57 55 54 53 52 51 50 49 48 47

II. Design of Rope Drums Drums for steel wire rope are made of cast iron. The drum diameter depens on the diameter of the rope. With a power drive the drum should always be provided with helical grooves (alur ulir) so that the rope winds up uniformly (melengkung secara merata) and is less subject to wear (tidak cepat aus) (Fig 57a).

The radius of the helical grooves should be selected so as to prevent jamming of the rope (the standard of the deep grooves for drums is listed in tabel 17).

There are 2 types of drum:

1. Drums with one coiling rope have only one helix (right hand)

2. Drums designed for two rope member are provided with two helices (right

hand and left hand).

The number of turn (jumlah lilitan) for one rope member is:

Where: i = transmission ratio of pulley system D = drum diameter H = height of the load rise Length of the helix on the drum (panjang lilitan/alur ulir) is: l = z s Full length of the drum:

1. For one coiling rope is: (

)

2. For two coiling rope is: (

)

Maximum compressive stress at the inner surface of the drum is:

Where: S = rope load w = wall thickness of the drum (tebal drum) = 0,02D + (0,6 ÷ 1 ) cm s = pitch length (lebar pich/lebar antar alur) l1 = jarak antara ujung alur kiri dan kanan pada drum (l1 = 0,25÷0,5 D)

Example problem: Sebuah electric overhead crane seperti gambar bekerja pada level medium duty dipakai mengangkat beban total Q = 10.000 kg (beban angkat + berat alat kait) dengan tinggi angkat H = 8 m. Jumlah tali penahan beban (rope parts) z = 6, efiseinsi total dari sistem puli adalah η =0,94. Bila tali yang dipilih adalah tipe Cross lay rope dengan jumlah kawat 222 buah, faktor keamanan K = 4 , kekuatan putus tali σb = 15.000 kg/cm2 dan E = 210 MPa. Tentukan: a. Diameter minimum tali kawat baja yang digunakan. b. Diameter drum, puli utama ,dan puli pembantu (compensating pulley). c. Jari-jari alur drum dan besarnya pitch alur drum. d. Jumlah lilitan/alur pada drum. e. Panjang total drum

Jawab: Beban pada setiap tali (tension in the rope) adalah:

Jumlah belokan pada sistem puli adalah n = 5. Dari tabel 7 untuk n = 5 didapat

Dmin/d = 26,5 a. Luas penampang tali minimum (dari rumus 13) didapat:

Diameter tali minimum adalah:

√

b. Diameter minimal drum dan puli utama adalah: D = 26,5 d = 26,5 x 10 mm = 265 mm (Bisa dipilih D = 300 mm) Diameter puli pembantu (compensating pulley) adalah: D1 = 0,6 x D = 0,6 x 300 mm = 180 mm (Bisa dipilih D1 = 200 mm) c. Jari-jari alur drum dan besarnya pitch (dari tabel 17) adalah: r1 = 7 mm (dipilih dari besarnya diameter tali d = 11 mm) s1 = 13 mm d. Jumlah lilitan/alur pada pada masing-masing sisi drum:

e. Panjang total drum

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