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Sep 2, 2015
- Buses bearing the three-pointed star from Turkey – a success story
- Body shop: Manhattan, robots and two cathedrals
- Paint shop: precision work in the electrocoat bath with a live current
- Assembly: from the empty shell to the finished bus
- Finish shop: comprehensive checks before the test drive
- By ship to Trieste and on from there under their own power
Stuttgart/Hoşdere – Some 3300 employees, about 4500 production units a year – the Mercedes-Benz bus plant in Hoşdere on the outskirts of the Turkish metropolis Istanbul is one of the largest of its kind in Europe. The plant produces four different model series, the Travego, Tourismo, Intouro and Conecto. The focus on top quality is striking: it is paramount in every step of the operation, from the welding robots and the detailed inspections to the extensive tests after finishing assembly.
Buses bearing the three-pointed star from Turkey – a success story
Mercedes-Benz Türk was founded in Istanbul in 1967 under the name “Otomarsan” with Daimler-Benz AG holding a 36 percent interest. The production of model O 302 buses started in 1968. In step with the growth potential of Turkey, the truck plant in the Aksaray Province in Central Anatolia began production operations in 1986. The new and generously dimensioned Mercedes-Benz bus plant in Hoşdere just outside Istanbul was built in 1994. The plant covers a total area of 360,000 m² with buildings occupying 139,000 m². The factory is a full plant for all steps in bus production and features a body shop, paint shop, assembly lines and finishing assembly. The plant is equipped with state-of-the-art manufacturing technology. It embodies the overall concept of a modern production process, which it realizes in ideal fashion. As the other bus plants, the Hoşdere plant follows the best practice examples in the integrated production network of Daimler Buses and also sets its own standards. Today, Daimler AG holds a 67 percent interest in Mercedes-Benz Türk. Since 1967, Daimler has invested around EUR 885 million in the Turkish bus production. The success speaks for itself: more than 75,000 buses bearing the three-pointed star have been built in Turkey to date. Currently, they are being exported to more than 70 countries around the world.
Body shop: Manhattan, robots and two cathedrals
Two welding robots, 86 CNC machines, 449 assembly jigs, 37 segment devices, cutting and punching machines, bending and bevelling machines, eccentric and hydraulic presses as well as two cathedrals – the body shop at the Mercedes-Benz Hoşdere bus plant assembles an impressive pool of machinery for processing steel and aluminium.
As in the other production departments of the plant, the employees here work in two shifts from 7:30 am to 11:30 pm, six days a week. To increase the capacity, both shifts were extended recently by an hour – this results in a production output of 16 buses a day.
As elsewhere in the manufacturing process, the principle of shop floor management applies: employees and managers meet daily at defined times in the production facilities to discuss the status quo of the production based on KPIs and solve any potential problems on site. Like a waterfall, this is done on different levels and leads to comprehensive information and fast reaction.
Manhattan – the joining station with a skyline
The body shells of the buses are built in segments, most of them even on in-house developed and manufactured jigs. One of the striking features is "Manhattan". This is where the front, middle and rear segments as well as the front and rear frames of the bus substructure are joined together. If you crouch slightly in front of Manhattan, you can discover a skyline that resembles that of the metropolis, which is how it got its name. Stations of this kind assist the employees in their complex job. The Hoşdere plant manufactures 33 different vehicle models of the four model series it produces. This does not even include the numerous special customer requests.
The substructure of a bus bearing the three-pointed star consists of three segments (front, middle and rear segment) plus front and rear frame. The tubes and sheet metals of the frame are inserted into special jigs and then welded together. As is customary in bus production, this is done without exception by hand. The shared production line is supplied according to the fish bone principle. Welding robots handle this difficult task on safety-relevant components in the substructure.
The tubes are cut to size by laser, among other things. Due to the variety, no less than three pairs of jigs are needed to build the side wall. The body shop is therefore able to handle the different heights and drops of the buses. All variants are built in the body shop on a shared line.
Cathedral: assembly of the components into a single unit
The frame of the substructure, front end, rear end and roof are joined into a single unit in the "cathedral". The name of this jig can be explained by its shape and size. Four employees weld the components together in two hours. For reasons of occupational safety and planning times, two cathedral jigs are in use in Hoşdere.
In the next step, the roof and side panels are installed on the adjacent body assembly line. In the interest of a smooth and homogeneous surface, the side walls are pre-stressed, the panels are glued to the frame and their ends are spot-welded. Subjects such as ergonomics and occupational safety rate highly in the production process and their rules are strictly followed. For example, welders don't just wear a protective helmet as usual, but are equipped with fresh-air automatic welding helmets.
Continuous inspections ensure high quality
Precision and checks in the body shop are important because the body shell forms the quality basis for the entire bus. That is why each welder puts a stamp on every component to attest to the proper execution of the work. The responsible foreman inspects two random vehicles during every shift.
Not only the body shell components are measured regularly at the individual stations, the cathedral itself is also measured repeatedly. Templates are used to check each vehicle while still on the production line to ensure the quality of the body shell. Once a month, an entire frame is checked in very detail as part of an audit. In the event of product modifications or new product start-ups, all new components are checked by laser for precise dimensional accuracy.
Flaps with rounded edges – invented in Hoşdere
The bus plant also manufactures the aluminium flaps and flap frames in-house. The edges of the flaps are not sharp, but rounded on both sides. Consequently, the paint adheres very well to the surface during later spraying –& nbsp;an important quality feature. The milling tool for rounding both sides of the flap edges was developed in Hoşdere and is now also being used in the body shop of the Mannheim plant. The flap frames and the outer skin of the flaps are only glued together after the surface treatment.
But first, each complete body shell is transported to the paint shop over an enclosed bridge with the help of an overhead conveyor. Another feature is also typical for the quality of buses made by Mercedes-Benz in Hoşdere: no bus leaves the buildings of the plant before it is completely finished and the buses are consequently protected against harmful environmental effects.
Paint shop: precision work in the electrocoat bath with a live current
In the paint shop, the frame weighing about four tonnes is placed on a steel table and rotated by 90 degrees. It is then pre-treated with a phosphate spray coating and afterwards dipped in an electrocoat bath (CDP). It ensures a continuous coat on all parts of the frame from inside and outside, which offers unsurpassed anti-corrosion protection. The CDP facility at the Mercedes-Benz Hoşdere bus plant is the first of its kind for bus production in Turkey and has already been in operation for eight years. By now, far more than 20,000 buses have passed through it.
The actual dip in the CDP facility follows after degreasing and applying a zinc phosphate coating. The particular vat is 16.5 m long, 4 m wide and 6.5 m deep. Pumps continuously circulate 390 tonnes of paint. The frame is dipped at an angle and removed at the reverse angle. This ensures that the protective paint reaches even the smallest cavity.
Individual control for each model
A special challenge the CDP facility faces is achieving the required different thicknesses: 12 µm on the inside compared with 20 - 30 µm (1 µm = one thousandth of a millimetre) on the outer surfaces. Across the various vehicles, this is only possible using a highly sophisticated strategy. For example, the program controlling the electric current and the tilt angles of the body shell are set individually for each station of the facility and for each model. In the case of a Travego, for example, the entire coating process takes 15 minutes.
Afterwards, the bus frame is washed in an ultra-filtration bath. While the substructure is being submerged, the upper section is washed by spraying down the frame. The body shell is then dried at high heat. This takes about an hour at an object temperature of 185°C.
Parts also go through the electrocoat bath
Not only complete bus frames pass through the electrocoat bath. Parts such as the aluminium flaps and flap frames as well as steel parts such as brackets or the electrics compartment are also pre-treated and painted separately. This pre-treatment is applied in two automatic spray booths: in the first booth, the parts are degreased with a sprayer, washed and activated, in the second booth they receive a zinc phosphate coating and are then washed again.
Before passing through the electrocoat bath, all steel parts are treated in a separate immersion phosphating facility. It comprises a number of steps such as two degreasing baths, washing, staining, zinc phosphate coating, passivating, drying.
The parts then reach the CDP facility and are treated together with the body shell. The sequence is set: first two body shells, then a load of small parts. After the electrocoat bath, the parts receive a grey/black powder coating in an automatic facility or are manually sprayed a different colour.
Flaps and frames are glued together in a jig. The same applies to the engine flap made of composite material: it is sanded and fitted with its frame. The top coat is only applied after these steps have been completed.
Seam sealing and underfloor protection
Before the top coat is applied to the body shell, the seams are sealed first. In addition, the front and rear section made of glass-fibre-reinforced plastic (GFRP) and the wheel housings are installed. After applying the polyurethane underfloor protection at a thickness of 1 mm, a filler is applied to the entire vehicle. It compensates for any surface flaws and at the same time serves as the adhesive surface for the top coat. The filler is dried and then sanded.
Top coat: some 450 colours and gradient colour paint finishes
The top coat is applied with a spray gun by specialists. This takes place in three spray booths under cleanroom conditions. A slight positive pressure in each booth prevents dust penetration.
Customers can choose from a virtually unlimited number of colours, starting with some 450 different paint colours. Many of them are metallic paints. Elaborate colour gradient paint finishes are popular for extravagantly styled touring coaches. Some vehicles require the use of ten or even 20 different hues. Markings – if they are not applied as decals – are also painted using stencils. The top coat is dried at an object temperature of about 60 - 90°C.
Hair-thin coat as protection and decoration
The overall thickness of the paintwork on a bus comprised of zinc phosphate coating, electroat, filler plus non-metallic or metallic paint is about 200 to 300 µm. For comparison: a human hair has a thickness of about 50 µm. Depending on the size of the vehicle, the weight of the entire paint coat is about 100 kg.
After this step is complete, the employees coat the luggage compartment, engine compartment and any driver sleeper compartment and/or the multi-purpose compartment.
Inspections under glaring lights
The fully painted bus is then inspected with eagle eyes under glaringly bright lights. Only then does the bus leave the paint shop and is taken to the assembly shop over another enclosed bridge.
Assembly shop: from the empty shell to the finished bus
The assembly shop at the Mercedes-Benz Hoşdere bus plant was converted from a box assembly to a line assembly process in 2009. Each of the more than 600 work stations was redefined as part of this change. The responsibility is divided between four teams of which each is in charge of different manufacturing areas.
The assembly line is U-shaped and runs in two or three parallel tracks. The buses move on electric transport carts, at first at a height of 120 cm. The transport systems were developed and even manufactured in-house at the plant.
Challenge: all buses on all lines
Each of the assembly lines builds all variants of the four bus model series. This requires the employees to have a high degree of flexibility and comprehensive knowledge. At the same time, the great variety of tasks at the individual stations necessitates precise planning of the processes.
Small parts are provisioned using the kanban system. So-called set carts carrying the majority of the remaining parts presorted by the warehouse arrive at their particular station exactly when needed. This avoids space-consuming and cluttered storage in the assembly shop.
U-shaped assembly line with 34 stations
Assembly operations are divided into 34 stations. They are complemented by four production islands for pre-assembly. They are located on the inside of the U-shaped assembly line and supply components over very short distances at the right time.
Quality is also paramount in assembly. As in the other manufacturing areas, the control mechanisms are divided into four independent quality circles: the employees check their own work and certify it with a stamp. The particular foremen check random samples. At the end of the assembly process, each vehicle is inspected on the basis of check lists. In the event of serious errors, the responsible foreman is called to the station by an andon light system to examine the error. The andon light system was introduced in the body shop two years ago and in the assembly shop in early 2012. It is an important element for tracing back and correcting errors in the production process immediately. The third circle consists of regular inspections by Quality Assurance in finishing assembly. In addition, there are regular and extensive audits.
Start in reverse
On the first leg of the production U, the buses move backwards through the assembly process. All parts are taken into the vehicle interior through the largest opening, the hole of the windscreen.
In the case of the touring coaches and the intercity buses, the main wiring harness with conduits and cables is installed in the centre of the vehicle. This is followed by the lavatory compartment, the installation of the roof-mounted air conditioning system and the insulation of the wall, roof and engine compartment.
At the same time, busy hands in pre-assembly bend the individual pipes for each bus, which are installed immediately afterwards. The next step is getting the bus ready for installing the powertrain, the auxiliary heater, the tanks and the front box.
A game of inches: engine and transmission are installed in the rear
Pre-assembly now prepares the plastic parts of the interior trim for installation. The same applies to the axles and the powertrain one station further down the line. The next components to be installed are the radiator and the front axle.
At the end of the first leg of the assembly U, the technicians manoeuvre the entire unit of engine and transmission into the rear end of the bus using a rolling device. This work station offers lots of elbow room and thus reveals the benefit of moving through the first part of the assembly process in reverse. The bus is fitted with its rear axle, the wheels are tightened with a multi-spindle screwdriver that tightens five wheel bolts to the defined torque simultaneously.
Three lines at the start of the interior work
The bus is turned around for the second part of the U-shaped assembly line and now moves with the front facing forward. In addition, the working height is lowered to 65 cm because work under the vehicle is complete. Furthermore, the assembly process now spreads out over three lines because some of the adhesive processes require longer idle times.
The floors are installed and pre-assembly prepares the luggage shelves and the interior ceiling. The employees there have to handle bulky parts. For example, the luggage shelf on the left side of the vehicle can be up to 12 m long. The exterior of the bus is fitted with bumpers and headlamps while the convector heater is installed inside.
The final metres with the easy mover
On the last leg of the assembly process, the buses run on their own wheels for the first time. They are moved along the two lines by a roller on one of the rear wheels, so-called easy movers that are driven by compressed air.
The technicians swivel the pre-assembled cockpit into the vehicle using a special device. Pre-assembly now prepares the windows: they are cleaned, masked off and coated with primer. The rear window and the side windows are moved with vacuum lifters and installed on the bus working from rear to front. Spacers ensure perfect positioning of the windows. On the inside, the kitchen and the bulkheads are up next.
Then come the seats. They are transported into the bus through the large hole of the windscreen with a conveyor. The seats are installed with a pre-set torque driver. The plant uses the same technology for other safety-relevant components, which eliminates the possibility of errors.
The engine starts for the first time
After this step is finished, the interior appointments have been installed completely. It is now time to install the windscreen using a vacuum lifter and an adhesive. Technicians connect the doors, program the control units and fill the fluids. An important première follows next: the engine of the new bus starts for the first time.
After fine adjustment of the doors, the bus leaves the building for the first time – but only to enter again through the nearest gate to check the proper functioning of the heating and air conditioning system in one of the two climate testing chambers. The wheel alignment and the entire vehicle are thoroughly checked next.
Finish shop: comprehensive checks before the test drive
But a bus from Mercedes-Benz is still a long way from being ready for delivery. In the finish shop, the employees first check the compressed air system and test the brakes. They also calibrate the speedometer. The vehicle is given a rough cleaning, carpeting is installed and the decals are applied. They are designed and prepared by the design centre of the finish shop.
Even though all production steps have already been inspected, the work of Quality Assurance now begins in detail. The inspectors perform the „pre-check“, a detailed check of the mechanical systems, interior appointments and electrical and electronics systems that takes almost seven hours to complete. All in all, the check lists fill 23 densely printed pages.
Touring coach or urban bus, premium vehicle or down-to-earth functional bus – all vehicles are examined equally meticulously using identical criteria. Any errors are marked and corrected immediately by the production workers. At Mercedes-Benz, even the inspectors are monitored – by analysing the results of the inspections.
Rain test and test drives on the testing ground and on the road
Afterwards, each bus is subjected to a 15-minute rain test. In this context, the term rain is certainly an understatement – it is actually a deluge. The employees go to great lengths to look for any leaks, even behind the open outer flaps after the end of the test.
Each bus is then driven on the company-own test rack. It simulates extreme twists as well as rough cross ruts and also includes a function test of the anti-locking braking system on an irrigated skid pad. This is followed by a test drive on public roads over a distance of 50 km on a defined route. But even then the tests are not done yet: the final inspection is up next.
Even the inspectors are monitored
However, that is still not enough for Mercedes-Benz at the Hoşdere bus plant: one vehicle from each model series is audited a month. This involves an extremely thorough inspection that takes two whole days. The Quality Assurance employees deliberately mime the inquiring look of the most discriminating customers. They are not only vigilant about function, but about the detail appearance as well.
To ensure that these eyes keep as sharp as ever and that a single standard is used, the Quality Assurance specialists from the various Daimler Buses plants meet once a year for a direct comparison.
The inspectors not only register individual defects, the plants analyse the results in detail. Monthly and yearly evaluations provide clear insights and lead to progress. All inspections of a bus are also documented. Even 15 years after delivery, the detailed delivery condition can be retrieved in case of a complaint.
By ship to Trieste and on from there under their own power
The Mercedes-Benz bus plant in Hoşdere is not only the largest bus manufacturer in Turkey, but also the largest bus exporter: some 75 percent of the production output is exported to more the 70 countries around the world. To Eastern Europe and other regions, the buses are delivered under their own power. Transport of the buses to Central Europe is first by ship from Istanbul to Trieste, Italy. From there, the buses continue on their own power. The odometer reads no more than 1500 km at the delivering service centre when the bus is delivered. That is where they are subjected to another pre-delivery inspection (PSI) before being handed over to the customer.