A Major Overhaul on a Two-Post Electro-Mechanical Vehicle Hoist
There are Electro-Mechanical and Electro Hydraulic Two Post hoists. I have decided to detail a major repair on an electro-mechanical two post hoist.
2.1 Overview of how an Electro-Mechanical Two Post Hoist Operates
I will try to explain the working mechanism of this hoist and will ask the reader to refer to the diagrams below as I do so. These hoists typically work on a nut (carrying and safety nuts) and bolt (spindle/lift screw) type system (Figure 2.3 below). They have two posts which are attached to a base (Figure 2.1 below). Inside the base is a gearbox on either side and a shaft which couples the two gearboxes together (Figure 2.5 below). The spindles are housed inside the posts. Attached to the posts are the carriages/saddles (Figure 2.1 below, No. 2) and attached to the saddles are the arms (Figure 2.1, No. 3). The saddles are also attached to the nuts. A lot of two post hoists work with a chain and sprockets (attached to the bottom of each spindle) which allows the motor to drive the spindles together. However, in my example, I will be using a Ravaglioli two-post hoist which uses two gearboxes and a shaft instead. The three phase electric motor which sits on the outside of the motor post (Figure 2.1, No. 5) has a triple pulley attached to it. To this pulley is fitted three V-belts which attach to another (larger in diameter) triple pulley which is in turn attached to a spindle. As electricity is supplied to the motor it turns the spindle via the pulleys and V-belts (Figure 2.2 below).
The bottom part of each spindle is attached to a gearbox, situated in the base, via a coupling. A shaft about 3 metres in length connects these gearboxes to one another (Figure 2.5 below). As the spindle on the motor post turns, it turns the shaft in the bottom of the base via the gearbox that it is attached to. The shaft now turns the spindle in the slave post via the gearbox that it is attached to. So, if the motor is activated, you will have the effect of both of these spindles turning. The spindles act as bolts which have a set of nuts attached to them. If we consider the motor post, if the motor turns the spindle clockwise the nuts will be raised up and the opposite will happen when the motor turns the spindle in an anti-clockwise direction. The saddles are attached to these nuts and the arms which the vehicle rests on are attached to the saddles and this is how the hoist either raises or lowers a vehicle.
Figure 2.1 An Electro-Mechanical Two Post Hoist
Figure 2.2 The Working Parts inside a Two-post-Hoist
Table 2.1 Part Names for Figure 2.2
A major overhaul on a two-post hoist would usually consist of one or more of the following:
1. Replacement of Carrying Nuts
2. Replacement of Safety Nuts
3. Replacement of Safety Features
4. Replacement of Thrust Bearings
5. Replacement of V-Belts
6. Replacement of Gearboxes, Shafts, Keys and Couplings
7. Replacement of Slides and Rollers
8. Replacement of Rubber lifting Pads
Let’s take a look at all of these in turn.
2.2 Carrying Nuts
The carrying nuts are made out of a non ferrous metal such as brass or a high performance plastic such as GSM or Vesconite. These are the hardest working items in the hoist because they not only “carry” the weight of the vehicle but do so with considerable friction, especially if there is a lack of lubrication in the system which does lead to premature wear and tear, rendering the hoist useless and possibly extremely dangerous if the safety system is not functioning properly. The nuts run on the spindle which is made of a hardened ferrous metal. Therefore it makes sense that the carrying nuts will wear out faster than most other parts on the hoist. All hoists have, or should have a safety system that either mechanically or electrically stops the hoist if the nuts wear out. On the hoist, such as the one shown in Figure 2.2 above, the safety catch (item 17) is shot out by the spring (item 18) as soon as the carrying nut wears out enough and falls onto the safety nut. Once the machine is let down onto the ground, the safety catch then catches onto a stopper inside the bottom of the post (item 28). This renders the hoist unusable only allowing it to go up about 30 cm off the ground – insufficient to raise a vehicle off the ground. On other hoists there is a lever between the safety and carrying nut. As the carrying nut wears out and falls towards the safety nut, this arm is pushed out from between the two nuts and simultaneously pulls on a wire cable that is connected to a safety switch. This has the effect of electrically shutting the machine down. The hoist can now move neither up nor down.
When I service or inspect a hoist and want to check whether or not the carrying nut is nearing the point of failure, there are various ways that I can do this. The hoist should be without a vehicle on it, although it is possible to check it with a car on the lift in some cases. Some manufacturers of vehicle hoists provide a gauge that you can insert between the carrying and safety nuts in order to check for wear. If the gauge does not fit between the two nuts it usually suggests that the carrying nut is sufficiently worn out to replace it. Another way that these nuts can be checked is to raise the saddle with a jack and to physically take the nut in my hands and lever it up and down. If there is a lot of play on the nut (air between the nut’s thread and the spindle) it would be a good indication that the part needs replacing.
Figure 2.3 The Spindle with the Carrying and Safety Nuts
2.3 Replacement of Safety Mechanisms
As stated above, all hoists have a safety mechanism that either mechanically or electrically stops the hoist if the carrying nut fails or if the transmission chain breaks or comes loose. However these items are generally not subject to high friction and hence wear and tear. There are some instances, however, when these might fail or require replacement. For example, the safety cable mentioned above could, over a period of time, wear out through continual use of the hoist. This will need to be replaced immediately. Item no 17 in Figure 2.2 above is a safety catch and is subject to quite a lot of pressure when it performs its function of catching on the stopper inside the post. Depending on what material this item has been made of (sometimes mild steel and sometimes cast iron) it may be bent and would then obviously need replacing.
Figure 2.4 Safety Catch, Sharing Pin, Nuts, etc
2.4 Replacement of Safety Nuts
It is very rare for this item to need replacing. After all, its only function is to trail the carrying nut, by way of the sharing pin (item 14 in Figure 2.2 above), when the hoist is being either raised and lowered. It generally carries no weight at all, excepting for the short period of time when the carrying nut has fallen onto it, until the hoist is mechanically or electrically shut down. An instance where one could look at replacing this item is when the carrying nut has fallen hard directly onto it in the manner described above. Usually however, the technician gets to replace the carrying nuts before this happens. Thus the safety nut usually bears very little stress and normally would not need replacing.
2.5 Replacement of Thrust Bearings.
Mention all the different types of bearings, e.g. tapered needle roller, etc. see types of Bearins in Metals folder. The thrust bearing is subject to a lot of stress and friction, but unlike the carrying nuts, (which are made of a non-ferrous metal) are made of an extremely hard metal. This item is greased up at every service interval and usually has a longer life span than the carrying nuts, for example. They may be replaced on a yearly basis. This is advisable as they can pack up without warning and usually cause downtime on the machine. Indeed, they are replaced at every major overhaul, along with the carrying nuts, etc. They fit into a housing (Item 8 in Figure 2.2 above) and the housing itself can be subjected to major damage if the bearing is not replaced immediately once it becomes noisy. It is quite easy to detect whether or not these items need replacing by using a stethoscope or even a screwdriver as a listening device.
2.6 Replacement of V-Belts
The V-belts will need replacing once they stretch enough, start fraying, or if they break. They are replaced rather often and at every major overhaul. It is not difficult to determine weather or not these need replacing because it is obvious to the eye.
2.7 Replacement of Gearboxes, Shafts, Keys and Couplings
The gearboxes (item 2 in Figure 2.5 below) are subject to a lot of friction. The hoist is constantly changing direction (up and down) and if there is any play between the cogs in the gearbox, there is a lot of strain put onto these cogs as the hoist changes direction. It is important to ensure that the gearboxes have always got sufficient oil levels in them at service intervals and at least an oil change at a major overhaul. The proper way to check for wear on the gearbox is to hold one side in one hand and to check for wear on the cogs (play) by turning clockwise or ant-clockwise. If I am turning, say, clockwise and find myself turning quite a while before there is resistance this is a sure indication that there is wear on the teeth and the gearboxes should be replaced.
The shaft, keys and couplings (Items 3, 4 and 6 respectively in Figure 2.5 below) all take a lot of strain, although this is lessened when there is very little to no wear and tear on them and made a lot worse if there is. Again, this is easy to check, by firstly holding either the shaft or coupling still with one hand and rotating the coupling/shaft back and forward with the other hand. Ideally there should be very little to not play. If there is, it will certainly get worse over time and quite rapidly too as the hoist constantly changes direction. Machining up of new or replacement of shafts, couplings or keys will be necessary in this case.
Figure 2.5 The Base, Gearboxes, Key, Couplings and Shaft.
2.8 Replacement of Slides/Spacers
The Slides (items 3, 20 and 21 in Figure 2.6 below) are also subject to a fair amount of friction which is minimised with proper lubrication. They are made out of nylon and the complete failure of one or more of these would result in a lot of damage to the posts (metal on metal), which would probably mean replacing the hoist. I will check these for wear either against a new one or with a vernier gauge. They must be checked and replaced if necessary at any service interval.
Figure 2.6 The Saddle with Spacers.
Replacement of Rubber Lifting Pads
The weight of the vehicle rests directly on the rubber pads (item 1 in figure 2.7 below). They act to prevent the vehicle moving while on the hoist and also to prevent the vehicle from slipping right off of the hoist. They also prevent damage to the vehicle and the hoist (rubber to metal instead of metal to metal). They must be thick, made of a highly durable/robust material (rubber or nylon) and must look “healthy”. They should be complete (i.e. no chunks missing) and must be securely fastened to the Supporting Plater (item 2 below). If they do not meet this criteria they must be replaced.