Issue 3/2006
05/02/06
Cheap or value for money - quality characteristics of lift doors
Dipl.-Ing. Thomas E. Lernet
The experience of lift planners and operators shows that the total cost of lift door components accounts on average for 10 % of the investment involved in installing a lift system. Surveys carried out among reputable specialist lift and building logistics planners also show that this area of the lift, i.e. the doors, is the cause of up to 90 per cent of all faults encountered in lift systems. The root causes are varied and are often linked to inappropriate treatment by users or other external influences. However, the primary reason why faults occur in lift doors is the use of what are considered to be low cost components.
Category: Issue 3/2006
Posted by: Editor
A general distinction is made between landing and car doors. The landing doors are permanently mounted in the elevator shaft on every floor, while the car door is a part of the car itself and consequently moves along the shaft. In new passenger lifts, horizontally sliding doors are used almost exclusively. Exceptions can occur in modernization projects and for lifts installed in existing buildings due to minimal lift shaft dimensions. Variants such as folding doors, segmental doors or roller doors are used here. In the case of highly stable freight elevators used in industry, rotary doors with flap lock are coming increasingly back into vogue, as these open automatically from a defined actuating force and so prevent damage by industrial trucks. According to EN 81 part 1 and 2, vertically sliding doors can also be installed in freight elevators.
As a rule, an elevator user only ever comes into contact with the visible assemblies of the door and the car. In the case of the doors, these are the opening and closing door panels, the permanently installed architrave and the threshold.
What is essentially the more complex section of the door generally remains concealed to the user, or is only exposed in, for instance, fully glazed lift shafts.
A horizontally sliding landing and cab door generally comprises the lintel, locking mechanism, guide rail, door suspension gear, bottom-guided door panels, threshold, architrave, trip dog and drive system.
The EC - type tested landing door locking mechanism and the majority of the kinematic components are located in the lintel.
The following quality attributes are of focal importance in determining the optimum, durable design of these components. A closed, welded or edged box profile guarantees a very high moment of resistance to torsion and deflection. This design offers benefits over a rolled, open type profile. It permits simple link-up to the guide rail on the face side, and helps prevent guide rail distortion. In addition, a lintel provides protection for the internal components over the entire length of the guide rail against soiling and damage. Fastening facilities over the entire width of the lintel provide an effective protection against deformation when the door is open, and also prevent heavy door panels from dropping.
To ensure a long service life for locking mechanisms, it is just as important to provide low-wear bearings such as roller or friction bearings for the moving components as it is to ensure an optimum structural design. Particular attention should be paid to ensuring that optimum transmission of force to the locking elements, i.e. the locking bolt, counter bolt and bolt latch, prevents deformation and moment occurring in the opening direction. For safety reasons, with centrally opening doors it is generally advisable to avoid locking action by means of the deflection cable. In case of failure of the connecting or telescope cables, no gaps or slits should occur between the two door panels or between the door panel and the architrave.
By using high-grade materials such as galvanized steel or stainless steel, it is possible to achieve an extremely long service life for the door suspension gear.
Particular attention should be paid here to ensuring simple, fast exchangeability of wearing parts such as rollers and counterpressure rollers. High strength resulting from FEM-optimized component geometry is particularly important for the door suspension gear at the closing edge. This ensures that the maximum expansion dimensions permitted in accordance with the state of the art are not exceeded even with very high, narrow door panels. Optimum positive locking fit against the guide rail is achieved using 2 rollers and 2 counter pressure rollers resting without play against the guide rail. This can only be achieved given minimal production tolerances for the guide rail profiles and the use of wear-resistant materials.
For standard applications, rollers and counter pressure rollers with low-maintenance plastic extrusion moulded roller bearings should be used. To address more stringent application requirements in terms of service life and noise characteristics, high-grade materials such as technical polyurethane and cast polyamide are used. These plastics demonstrate minimal flattening characteristics and a long service life coupled with high notched impact strength. Precise coordination between the guide rail and roller geometry results in lower surface pressure and so results in a noticeably lower roller noise and also reduced wear.
In comparison with a guide rail in extruded aluminium, the use of a wear-proof steel guide rail is far more cost-intensive but at the same time ensures many times the service life. With a modulus of elasticity 300 per cent higher, it also brings about a considerably higher moment of resistance from the same component dimensions. Over recent years, newly developed production techniques have permitted extremely high quality running surfaces to be economically produced, which greatly enhance door running convenience. When designing a door, attention should also be paid to ensuring that the guide rail can be separately exchanged in case of damage. This reduces repair costs and also outage time compared to other design variants in which the guide rail is an integral part of the lintel.
When a lift is operated correctly, the lift door panels are the moving components of a lift which come into direct contact with users. To ensure that the prescribed degree of static strength is achieved and to eliminate any possibility of falls from a height, depending on the design of the door panels - whether sheet metal, frame construction or all glass - a series of standards have to be adhered to. The door panels are also viewed as a part of the locking mechanism and are tested as part of the EC type testing procedure, for example by the TÜV Gruppe Süd test inspectorate, using the expansion force of 1000 N required according to EN 81 part 1 and 2 at one meter above the threshold. A number of relevant standards such as DIN EN 81 Parts 1 and 2, DIN EN 81 Part 58, DIN EN 81 Part 71, DIN EN 12600, DIN EN 18091, TRAV have to be adhered to, depending on the specification. An essential factor is the execution of pendulum impact tests, which quickly and clearly reveal the weak points of a door. Equally as important as the configuration of the door panel are also the suspension gear connections and the bottom guides.
Design solutions which stipulate connection of the door panel and suspension gear by pinning only very rarely permit quick and simple assembly. Infinitely variable height and depth adjustment of the door panels is already possible in many designs but this is not provided as a matter of course. Bottom guides over the entire width of the door panel considerably restrict lateral play of the panel, and depending on their design also offer facility for substantially improved prevention of falls from a height and for the integration of restraint systems of the type required by EN 81 - 71 for categories 1 and 2.
In order to reduce maintenance costs, exchange of the bottom guide should be quickly and easily possible without removal of the door panel. To reduce door running noise, special sliding materials or high-performance technical felts can be used.
In order to reduce the risk of injury from glass door panels, special attention should be paid to ensure flush fitting between the sheet metal and glass surface. Although this is frequently taken into consideration in frame construction glass door panels, due to both technical and economic considerations it is neglected in the case of glass door panels contained only at the top and bottom. In order to achieve a perfect flush fit, the pre-tensioned laminated glass panel made up of toughened safety glass panes must be recessed by milling or sanding on the user side by the thickness of the sheet metal used to create the closing edge protective trim.
Because this technology entails high reject rates and cannot be performed subsequently once the door panels have been mounted, in the majority of cases closing edge protective trim is simply mounted over the top.
The situation is similar in the case of all-glass door panels made of several individual toughened safety glass panes. To achieve a smooth, stepless closing edge between the individual glass panes, the laminated glass panel needs to be subsequently ground and polished.
The best course of action here is to arrive at the necessary agreements with the manufacturer in advance, as this optically highly attractive design entails additional costs but is not stipulated in any standard or directive. To reduce the door recess gap, particularly in telescopic doors with all-glass door panels, cheap Plexiglas rods are frequently used in preference to glass rods. Requesting a manufacturer’s confirmation of UV resistance and long-term resistance to cleaning agents can save considerable costs and aggravation later on in the event of subsequent complaints.
To help prevent damage in case of minor knocks from suitcases or shopping trolleys against the architrave, the sheet metal should be at least 1.5 mm or better still 2.5 mm in thickness. Regrettably in standard applications it is not uncommon for suppliers to provide sheet thicknesses of under 1 mm. By providing small edge radii or integrated deflection systems such as brushes or safety edges in the design of the architrave at the door recess gap, it is possible to substantially reduce the danger of children’s hands being drawn into the gap.
By optimizing the architrave geometry, it is also possible to prevent the possibility of cutting injuries when pulling back trapped fingers.
There are also a whole range of different designs and variants possible for the door threshold.
In order to minimize the risk of stumbling for passengers stepping over the lift threshold, the guide slot required for the lower door panel guide should be designed as narrow as possible. The widths offered by manufacturers fluctuate anywhere between 7 m and 16 mm.
Narrow slots reduce the incidence of collected dirt, with the added benefit of lengthening the necessary cleaning intervals. An extremely low-maintenance threshold version is the concealed bottom guide, as this permits the guide slot to be completely dispensed with in the visible area. To prevent shearing points when readjusting the lift car, particular attention must be paid here to ensuring that the slot required for concealed door panel guidance is placed at least 60 mm below the upper edge of the threshold. The minimum material thickness of the step edge should 3 mm to account for wear. For outdoor applications, special slip-proof surfaces and threshold heating systems are offered. In industrial applications, precisely the opposite strategy is applied in the case of large-scale freight lifts. Here, wide guide slots with generously designed dirt holes and sufficient play between the guide and threshold are preferred, although the door recess gaps specified by EN 81 Part 1 and 2 have to be adhered to. Loading and unloading by fork lifts without due regard for proper procedures can involve an increase risk of lift defects, as increased burr forms at the threshold profiles, causing the doors to jam. Threshold profiles chamfered beforehand to below 45° provide an effective method of preventing this phenomenon.
The same quality criteria apply to car sliding door components when it comes to design of the lintels, suspension gear, running and counter rollers, guide rails, door panel and threshold as to landing sliding doors, although here the issue of assessing door drive systems and trip dogs also has to be considered.
In the standard range of door panels up to 400 kg, low-maintenance DC motors or small frequency-controlled drive systems are frequently used. For optical reasons, particularly in the case of glass shafts and to protect against damage and soiling, it is beneficial to arrange these inside the lintel.
The incremental encoder used to detect the door position should be mounted here for the same reasons. In the case of high-capacity lifts, powerful frequency controlled direct drive or crank drive systems are frequently used. The noise levels of these technically sophisticated and high-quality drive systems are generally between 50 dB(A) and 55 dB(A). The acceleration and braking action of door movements should be capable of independent and adjustment using potentiometers. Attention should be paid to ensure that the door control system does not execute any automatic door movements without a command from the higher-level lift control system. Modern door controls already feature several permanently pre-installed programs which permit situation-dependent adaptation of door movements for applications as diverse as old people’s homes and office buildings. Some suppliers also offer facility to select different door opening widths depending on the landing. In order to avoid creating unwanted dependency on any one manufacturer or supplier, planners and users should ensure that the door drive and control systems used are freely available in the components market.
In many applications, the door movement of the car door is transmitted by the trip dog to the landing door. To ensure maximum door running convenience, it is important to ensure that before the first door movement is initiated, the trip dog opens the locking mechanism of the landing door and has created a positive locking connection to the landing door. This is achieved by providing a design in which the two trip dog brackets carry out a relative movement apart from each other during the unlocking phase and towards each other during the locking phase. This ensures that the landing door is always completely closed and that the locking action is a con-trolled one. This effectively eliminates malfunctions due to open landing doors as a result of dirt in the threshold or exposure to wind loads. Mounting the individual moving and power transmitting elements in double deep groove ball bearings or needle bearings is just as essential to a long service life as the use of top-grade materials such as galvanized sheet steel or stainless steel. Although injection moulded aluminium parts and trip dogs mounted in plastic bushes are also widespread in practical application, these offer a substantially reduced service life and are also far more susceptible to breakage. If a mechanical car door locking mechanism can be subsequently attached at the trip dog, then it may be possible to dispense with costly retrofitting of locking systems or costly shaft wall cladding solutions.
Summary
Users and planners can themselves actively influence the quality of a lift and the follow-on costs incurred during operation and maintenance by precise specification and selection of individual lift components in their invitation to tender. In contrast to supposedly cheap solutions, operators begin to save money as soon as the warranty has expired: Quality materials last longer, work more reliably, are less susceptible to defects and less likely to require repairs and retain their value appeal for longer. This increased reliability is in evidence in day-to-day operation, and clearly confirms that the “cheaper” option generally ends up costing more.
3/2006
