Bogies let a locomotive's long body negotiate curves. Bogie wheel base — the distance between the centres of a single bogie's extreme wheels — must be well-proportioned: small enough to negotiate curves, but large enough to prevent tipping and jerking. A locomotive always has two or more bogies on which the body is mounted; the distance between the centres of extreme wheels across the whole locomotive is the total wheel base.
Bogie Classification — Wheel Arrangement
Bogies are classified by number of axles and type of axle drive — together called the wheel arrangement, designated B, Bo, or Co. A locomotive's overall wheel arrangement is written as B-B, Bo-Bo, or Co-Co (always two-or-more bogies).
| Code | Axles | Drive Type |
|---|---|---|
| B | Two | Axles mechanically coupled |
| Bo | Two | Axles independently driven |
| Co | Three | Axles independently driven |
| Bogie Type | Loco Type | Characteristics |
|---|---|---|
| B-B | WAG-1, WAG-4 | One traction motor per bogie drives two wheel sets in that bogie |
| Bo-Bo | WAG-2 | Two traction motors per bogie, each motor drives an individual wheel set |
| Co-Co | WAM-4, WAP-4, WAG-5, WAG-7 | Three traction motors per bogie, each drives an individual wheel set |
| Co-Co Tri-mount | — | 3 traction motors drive 3 wheel sets each; loco body rests on the bogie at 3 triangularly-displaced points — the centre leading point carries 60% of body weight, the other 2 points (at High Dynamic Load) carry 20% each (20+20=40% total) |
| Co-Co Flexicoil | WAP-4, WAP-7, WAG-9 | Each bogie has a double (primary + secondary) suspension system |
Co-Co Tri-mount Bogie — Real Design Detail
Fitted to the majority of Indian Railways locomotives. A single-piece cast-steel bogie frame carries the centre pivot in the cross-member toward the loco's end. The centre pivot carries 60% of the vertical load and is the sole path for tractive/braking forces; the remaining 40% of vertical load is carried by two side bearers, which carry no tractive or braking force. The frame is supported by four sets of double equalizers running from the end axles to the centre axle, with full equalization achieved by suitably positioning the springs and controlling their working height. The loco-body weight reaches the bogie at the centre pivot plus the two side bearers — a three-point support, hence 'tri-mount' bogie. Because two traction motors must be supported from one transom while keeping overall bogie length to a minimum, the tri-mount bogie's wheel spacing is unequal (relatively small wheel base).
WAP-4 Real Profile — Production History & Full Technical Specification
WAP-4 was developed after the earlier WAP-1 class proved inadequate for the longer, heavier (24-26 coach) express trains becoming Indian Railways' mainstay. Introduced in 1994 with a similar bodyshell to WAP-1 but Hitachi traction motors developing 5000hp (5350hp starting), it uses traditional DC-loco-type tap changers driving 6 traction motors in Co-Co fashion. Over 800 units entered service, homed at 13 sheds; WAP-1, WAP-3, and WAP-6 units were later rebuilt to WAP-4 specification by replacing bogies and electricals. Newer units added microprocessor-controlled diagnostics, static converters (replacing Arno), and roof-mounted dynamic (rheostatic) brakes.
Design evolution: the first ~150 units had waist-level headlamps in a protruding nacelle; later units placed the headlamp in a recessed nacelle; from road #22579 onward, headlamps moved to the roof-top. Newer locos also got larger windshields, more spacious cabs with bucket seats and ergonomic controls, and a mix of digital/analog control-panel displays (all-analog in older units). Numbered from 22069 to 22980.
| Parameter | WAP-4 Real Value |
|---|---|
| Gauge | 5 ft 6 in (1676 mm) |
| Top speed | Service 140 km/h; test runs 180 km/h (achieved 169 km/h in trials) |
| Power output | 5350 hp (3989 kW) starting; 5000 hp continuous |
| Starting Tractive Effort | 32000 kgf |
| Manufacturer | Chittaranjan Locomotive Works (CLW) |
| Traction Motors | Hitachi HS15250 |
| Gear Ratio | 23:58 (one loco, #22559, reportedly has 23:59) |
| Transformer | 5400 kVA, 32 taps |
| Axle load | 18.8 t |
| Loco weight | 113 t (original units 120 t, reduced via lighter materials) |
| Bogies | Co-Co Flexicoil Mark 1 cast bogies; primary + secondary wheel springs with bolsters |
| Pantographs | Two — high-speed Stone India (Calcutta) AM-92 and CCPL IR-03H |
| Current ratings | 1000 A for 10 min; 900 A continuous |
Conventional (Flexicoil) Bogie & Its Components
A bogie (or truck) is a wheeled chassis/framework carrying wheels, attached to a vehicle as a modular subassembly of wheels and axles. The WAP-4's Flexicoil Mark-1 bogie's frame and bolster are steel-casting box-type construction per RDSO standards. Loco body weight transfers to the bolster through the centre pivot. The steel-cast H-type bolster sits on the steel-cast bogie frame at four corners, via pairs of helical springs in spring pockets of the frame's main longitudinal member; the bolster is located relative to the frame by upright pedestals integral to the frame, transmitting force both ways. Spring-loaded sunbeam pistons (2 per bogie, phenolic material) provide high friction between bolster and frame for damping in both vertical and lateral oscillation. Lateral stops on both bolster and frame limit side movement (flexible action, order of 32 mm). The frame itself rests on the axles via another spring set on the axle boxes. The locomotive superstructure's load rests on the centre-pivot bowl, fitted with vertical/horizontal Fluon (Vx2) liners giving rotational freedom between body and bogie. Two diagonally-opposite lifting links ease bogie installation/removal.
- Traction Motor: HITACHI HS-15250A axle-hung, nose-suspended type, mounted via suspension tubes with taper roller bearings; all axles power-driven; air-cooled by external blowers (motors run on pulsating current).
- Gear Case: houses and protects the traction motor's main bull gear and pinion from foreign material (dust/stones/water); holds lubricating oil for the bull gear-pinion relative motion.
- Nose Suspension Drive: the drive mechanism for electric railcars — roughly half the traction motor's weight is carried by the wheel axle via the suspension limit, the rest by the bogie frame via the motor frame's 'nose'. The suspension limit permits no relative movement (except very limited) between wheel axle and motor axle, letting the two be directly gear-connected.
- Brake Rigging: distributes braking force from the brake cylinder to the various wheels via rods/levers suspended from the underframe and bogies, linked with pins and bushes. Requires careful setup and regular adjustment — badly-set rigging causes wheel flats or inadequate brake force.
- Brake Cylinder: six (203mm dia × 203mm stroke) brake cylinders per bogie operate clasp-type brake rigging. Each cylinder piston connects to a brake lever actuating one wheel's brake only; in-service brake-shoe adjustment uses an adjusting rod at the bottom. Brake blocks/shoes are of conventional type.
- Wheelset: the wheel-axle assembly together with the bull gear. Two real bearing arrangements shown: MSU-type (bearings on either end) and the TA0569TM axle-bearing type.
Weight Transfer & Mechanical Power Transfer Chain
WAP-4 weight-transfer path (real, to provide axle load for adhesion): loco body rests at the bolster's centre pivot → bolster rests on secondary springs at 4 places → secondary springs seat on bogie frame → bogie frame rests on primary suspension springs → primary springs seat on axle box → axle-to-wheel weight transfer provides the axle load needed for adhesion.
- 1. Traction motor rotates on DC input power, delivering mechanical output at its pinion.
- 2. Pinion engages the axle's bull gear; the axle rotates as the bull gear rotates.
- 3. As the axle rotates, so does the wheel (with the axle box on the axle); rotational forces convert to linear forces through rail-wheel friction.
- 4. These linear forces shift to the bogie through the bogie pedestal at both sides of the axle box.
- 5. From bogie frame to bolster, through the pedestal at the friction-damper housing.
- 6. Forces shift from bolster to centre pivot, then centre pivot to loco body.
- 7. From loco body to the centre buffer coupler, hauling one train/formation.
Wheel Profile Table — Real Condemning Limits
| Parameter | New | Condemn | Wear |
|---|---|---|---|
| Root Thickness (RT) | 37.5 mm | 31.5 mm | 6 mm |
| Flange Thickness (FT) | 32 mm | 29 mm | 3 mm |
| Tread Wear (TW) | — | 6.5 mm max | — |
| Wheel Diameter | 1092 mm | 1016 mm | 81 mm |
| Wheel Gauge | 1676 mm | — | — |
Wheel Materials & Real Diameter-Variation Limits
Some locos use renewable steel tyres per specification IRS-R15-64 (oil-quenched, tensile strength 100-110 kg/mm²); others use a solid wheel disc conforming to IRS M2-65 and IS 1030-1962. After assembly, wheels are inspected for both diameter and profile. New wheel-disc diameter for WAG5/WAM4/WAP-type locos is 1090mm; condemning diameter is 1016mm.
| Maximum Diameter Variation Between Two Wheels | New | In Service |
|---|---|---|
| On the same axle | 0.5 mm | 1.5 mm |
| On two axles of the same bogie | 2.0 mm | 5.0 mm |
| On two axles of different bogies of the same loco | 15.0 mm | 20.0 mm |
Wheel-profile variation happens through root wear and flange wear, both checked with a condemning gauge. Permissible maximum clearance from the gauge: root wear (checked at position Y) — 6mm; flange wear (checked at position X) — 3mm — matching the Root Thickness/Flange Thickness wear figures already in the table above.
- Definitions: suspension between axle and bogie frame = primary suspension; suspension between bogie frame and vehicle body = secondary suspension. Purpose: reduces track-induced vibration, balances vertical loads between wheels, improves ride comfort.
- Co-Co tri-mount bogies (WAM4, WAG5) have ONLY primary suspension: 4 groups of helical coil springs, each group with 2 nests (one outer, one inner coil). To prevent uncontrolled body bounce, dampers resist the spring action — friction dampers or hydraulic dampers. Tri-mount bogies use friction dampers/snubbers on 4 of the inner coils per bogie, damping vertically via friction between two co-axial cylinders.
- Flexicoil Mark I bogie (WAP1) has TWO stages of vertical suspension using helical springs — primary (axle box to bogie frame) and secondary (bogie frame to bolster). Transverse flexibility between body and bogie comes from the flexicoil action of the secondary-stage helical spring; bolster-spring supports sit on a wider arm for better rolling stability.
- Flexicoil Mark II (WAP3) — a further speed-oriented modification: 4 vertical hydraulic dampers in secondary suspension + 6 friction dampers in primary suspension per bogie (2 per axle), plus 2 hydraulic dampers per bogie fitted laterally between body and bogie frame for transverse damping.
- Flexicoil vs tri-mount: oscillation trials showed the tri-mount bogie's speed potential is limited by single-stage suspension and absent lateral flexibility. In the flexicoil design, the bolster (body rests here) sits on heavy coil springs and floats independent of the bogie frame — springs provide vertical cushioning AND flex laterally, letting the bolster move sideways; this fully-floating frame eliminates the need for an equalizer bar. The fabricated flexicoil bogie has both primary and secondary helical suspension, with 2 friction dampers between bolster and bogie frame (secondary suspension) for vertical/transverse damping.
Distinctive Bogie Designs by Loco Class
- WAG6A: three fabricated bogies in a Bo-Bo-Bo arrangement. Bogies 1 and 3 are identical; the intermediate Bogie 2 is specially designed to allow lateral displacement in curves.
- WAG-7 — a distinctly bolster-LESS bogie design: 3-axle, two-stage suspension, floating pivot, unidirectional axle-hung nose-suspended traction motors. Frame is straight, fabricated box-type with three transoms carrying nose suspension. Body weight is supported on FOUR rubber side bearers mounted directly on bogie side beams (shims below the outer side bearer distribute load in a real 60:40 ratio — 60% on side bearers adjusted to prevent load reaching the centre pivot, 40% on the remaining two side bearers). The centre pivot carries NO vertical load at all — used only to transfer traction/braking forces. The bogie frame is supported on the axle via helical springs mounted on an equalizer beam; the equalising mechanism (equalizers hung directly on end axle boxes, supported on the middle axle box through a link and compensating beam) achieves equal axle loads on uneven track and reduces weight transfer at start.
WAP5 & WAG9 3-Phase Bogies — Real Design Features (ABB Henschel)
WAP5 and WAG9 bogies were designed by M/s ABB Henschel, Germany, and manufactured in ABB's factory in Australia. WAP5 uses a 2-axle BOLSTER-LESS Bo-Bo bogie with fully-suspended traction motors (reducing unsprung mass) and 2 stages of suspension: the loco body rests on 2 pairs of secondary helical springs on bogie long beams; the bogie frame is supported on primary helical springs resting on the axle boxes. Transverse flexibility between body and bogie comes from the flexi-coil action of the secondary springs. Hydraulic dampers work in vertical+lateral modes at the secondary stage (between underframe and bogie frame) and vertical mode at the primary stage (between axle box and bogie frame); 2 dampers between loco body and bogie give constant damping force in longitudinal mode, and one damper per traction motor cushions the motor's lateral motion. Traction links (bogie frame to axle boxes) transfer traction forces from axle to bogie frame; low traction bars then transmit these forces from bogie to underframe — connected toward the underframe's middle specifically to improve cab riding behaviour.
WAG9's Co-Co bogie is a 3-axle, 3-motor assembly; 2 bogies support the loco's entire weight and transmit tractive effort to the rails. Real suspension terminology: primary suspension ('Flexi coil' arrangement — twin coil springs on the axle journal box fore/aft of the axle line, with vertical hydraulic dampers damping spring rebound) permits lateral axle movement; guide rods between axle journal boxes and bogie frame (with Spheribloc rubber bushes allowing lateral movement without undue restriction) provide longitudinal control and transmit tractive/braking effort to the bogie frame. Secondary suspension (coil springs + vertical hydraulic dampers between bogie frame and underframe) carries the loco body's weight via a 'Flexi Float' arrangement allowing the car body to move both laterally and vertically within limits relative to the bogies. Real damper set: lateral buffers/dampers limit lateral movement amount/rate; rebound-limit and vertical dampers limit vertical rebound amount/rate; yaw (longitudinal) dampers control car-body pitch rate; guide rods control axle-to-bogie-frame fore-aft movement, while a link rod controls bogie-to-car-body fore-aft movement.
Both WAP5 and WAG9 carry 4 real pneumatic braking systems: automatic train brake, direct loco brake, parking brake, and anti-spin brake. As with tractive effort, braking effort transmits to the bogie frame via the axle journal boxes and guide rods, then from bogie frame to locomotive via the traction rods. WAP5 additionally has disc brakes for the improved braking performance high-speed operation needs.
- 1&12 Sanding box, 2&5 Wheel flange lubrication reservoir, 3 Primary suspension damper, 4 Secondary suspension yaw damper, 6 Secondary suspension spring, 7 Safety chain, 8 Secondary suspension vertical damper, 9 Horizontal damper, 10 Wheel set guide, 11 Bogie frame.
- 13 Wheel set, 14 Wheel, 15 Brake blocks, 16 Brake lever, 17 Brake cylinder, 18 Brake rod, 19 Primary suspension spring, 20 Wheel flange, 21 Sanding pipe, 22 Traction link.