Your guide rails play a crucial role in the safety of your goods hoist. If your guide rail setup is insufficient, the goods hoist is naturally non-compliant. Even worse is that your safety gear, the last line of defence in an emergency, is unlikely to be fully functional. In the best case, you might just have an illegal setup or perhaps damage the rails by deforming them beyond their elastic limits during an emergency safety gear activation (still a costly correction). In the worst case, your goods hoist could free-fall to the bottom of your shaft because the guides ripped out of the wall or because the safety gear couldn’t maintain contact with the severely deformed rails. In that case, you are generally going to need to replace the goods hoist, correct the issue with the guide rail setup and also deal with a lot of tough questions and H&S consequences.
So how do you know if your guide rails are suitable for the loads your unit will experience during safety activation? Well, you don’t. The lift inspector on your project, in general, won’t be able to verify it for you either. In most cases, he/she should notice if something is highly suspicious and then query it. There is quite a high chance of this slipping through. Ultimately, you have to trust your chosen supplier or specifically request external verification if you are suspicious of a potential issue e.g. after a concerning and awkward meeting where your chosen lift supplier was unable to convincingly provide and explain the static and dynamic force details.
A natural thought in response to this concern is to just test the guide rails, right? In an ideal world, yes! However, the loads experienced during normal use usually don’t determine the spec of your guide rail. For heavy-duty units, especially those loaded by a forklift, the forces experienced during loading/unloading are sometimes the worst that will be experienced, which would make it feasible to test to some extent. Most of the time though, guide rails are specified based on the dynamic loads experienced during emergency safety gear activation with an offset load in the goods hoist, and this is more problematic to test.
Depending on whether your unit uses progressive or instantaneous type safety gear, the impact factor due to the sudden deceleration of your falling lift is two or three respectively. This impact factor, along with certain car dimensions, the vertical distance between car guide shoes, the vertical distance between guide brackets, the capacity, car mass, the position of the car’s centre of mass vs the guide rails vs the suspension points, along with a couple of other values all have to be factored into the calculations. Fortunately, the required formulae for most cases are conveniently detailed for us in SANS 50081-50:2017 and in our older lift regulations as well. Guide rail stress and guide rail deflection must be checked based on these formulae. It’s a great comfort to know that these have been put together for us, firstly to make it safe to verify guide rail suitability and secondly so we know what safety factor is actually in place.
Another concern is that many people, even in the lift industry, are surprised that there are horizontal loads applied on lift guide rails that are transferred to building walls. We have seen many drawings that either don’t detail the horizontal loads at all or that detail them only in part or just incorrectly. This is especially concerning on large units. Unfortunately, most people see the vertical loads, the ones everyone expects and then proceed based on the belief that the loads have been fully and correctly detailed for them. The horizontal loads, as well as the distance from the side walls to the guide rails, absolutely must be communicated to the structural engineer that is taking responsibility for or designing the lift shaft.
The distance from the guide rails to the side walls might seem irrelevant at first glance, but if you think about it for a moment, this position has a significant impact on the “pull out” forces at the fixing points between the guide brackets and the wall itself. Even just in normal use (not a safety activation), picture the situation where the user places a 2-ton load near the back of a goods hoist. To make it easier to picture the effect, let’s say the car is 3m deep, and that the 2-ton load is on a 1m x 1.2m pallet that is almost touching the back panels of the car. In this case, the entire 2-ton load is positioned on the far side of the car centre line, which we will assume is at 1.5m for simplicity, and that the guide rails and rope suspension points are positioned on this 1.5m centre line as well. That 2-ton load will make the back of the car drop slightly compared to the front of the car. If not for the car’s guide shoes which interface with the guide rails above and below the accessible cabin, the car would be completely unable to support the load, instead violently rotating until it wedges in the shaft or the 2-ton load falls through the back car panels and out of the car. The horizontal force supported by the guide rails, applied through the car guide shoes, counteracts the moment due to unbalanced loads in the car.
The guide rails are supported by guide brackets which are attached to the building walls. If either the guide shoes or the guide brackets are too weak to handle the loads, we have a big problem. Let’s assume that the brackets and shoes are well designed. Now we have a horizontal load applied on the guide rail through the guide shoes. To prevent the guide rail from shifting, we have guide brackets to transfer the load to the wall. If the distance from the guide rail to the shaft wall is small, say 100mm, the pull-out forces experienced by the bracket-to-wall fixing points that are countering the moment caused by the 100mm load offset are not severe. However, if the distance was say 500mm, the pull-out forces could become problematic unless the guide bracket design was adjusted appropriately so that the wall anchors could cope with the load. In general, brick walls are concerning when these pull-out forces are large (due to load, car dimensions, and/or guide rail vs wall positions). Suitable concrete pads, or even better, full concrete shafts are strongly recommended and often unavoidable.
Another common thought is that if the existing building was suitable for the old lift, it will surely be fine for the new one. This thought process is reasonable, but only if the fully detailed loads are compared to ensure that they match and provided that the original structure was approved by a suitably qualified structural engineer. For goods hoists in particular, adding safeties on installations that had none before (quite common before our regulations were improved) will radically change the loads experienced by the building. Even if the original unit did have safeties, a full load activation with an offset load position in the car at just the wrong spot in the lift shaft may not have happened yet, creating a false sense of security. Considering that guide rail stress and deflection, in multiple load cases, must be calculated for every lift to confirm that it is functional and compliant. There is no excuse for these loads to be incorrect, incomplete or missing entirely. The considerations discussed above are not exhaustive, but they are sufficient to highlight the importance of a proper analysis of every goods hoist’s guide rail setup. Goods hoists typically apply high loads to their guide rails, which are a crucial part of the safety system that protects the user in an emergency. If you would like advice on this topic, please feel free to contact our engineers for assistance – they are more than happy to help!