Análisis de contexto 1. Orientación

One of the most difficult tasks you’ll face when building a Shaker Table is designing the motive power system and finding just the right electric motor to drive the equipment. Larger tables are easier to do than smaller ones because for the bigger tables you can just use almost any one-half horsepower A/C motor and these are fairly well standardized with respect to frame mounting dimensions and available pulley selections. These types of motors come in several different ‘frame’ designations. The most common that you’ll find in hardware stores is called the NEMA 56 ‘base-mount’ frame type that has an integral mounting frame. This is the type of motor you’ll find driving drill presses, bench grinders, band saws, air compressors and a wide variety of shop power tools.

The other type of NEMA 56 motor is called the 56-C ‘face mount’ and these go all the way down to one-forth horsepower and are generally much smaller in external dimensions. They are designed to bolt onto a fabricated mount or to another piece of

equipment like a fan cage or a direct drive for a bench mounted belt sander. There is also the NEMA 48 base mount that has a slightly smaller footprint.

Any of these motors with various mount types are available from almost any industrial supplier such as Grainger’s or McMaster-Carr but they aren’t inexpensive and having to buy a new motor is where most home-built Shaker Table projects get relegated to the sideline.

This is where ‘scrounging’ and ‘cannibalism’ really become important. You can usually find a huge variety of used motors at swap meets, yard sales, junk stores, salvage yards and used appliance stores. Sometimes you can buy an entire piece of equipment with a good motor in it for next to nothing and then reuse the motor and throw all of the other parts in the trash. I’m constantly on the look out for motors and pumps at garage sales so don’t buy a new one unless you absolutely have to.

One of the ideal equipment candidates for being cannibalized is any old diaphragm water pump as these usually have very durable but small motors and some kind of eccentric drive mechanism all in one nice housing.

Regardless of what you end up with as a power plant there is usually little doubt that it’ll be running at either 1140 or 1725-40 rpm’s. Of course the Shaker table only needs between 200 and 300 cycles per minute so you’ll end up with some kind of pulley and v- belt system to reduce the rpm’s between the motor and drive shaft. If you luck out you might end up with a variable speed motor and then you could go with some kind of direct drive but this is unusual. You can buy a speed controller for most motors but then again they are relatively expensive.

During the mockup stage you can use a regular old variable speed electric drill to drive your table as they can be set to run in the rpm range you’ll need. In practice you use some electrical tape or a hose clamp to keep the trigger depressed at the speed range position you want. Large hose clamps work really well because you can adjust the speed range using the clamp screw.

Assuming that you’re on a budget and find a good electric motor that typically will be running between 1725 and 1740 rpm we need to find a way to reduce this speed to drive our eccentric shaft to provide movement to the table deck support frame at around 290 to 300 rpm.

The easiest and cheapest way to do this is with a system of pillow block arbors and different sized pulleys.

There are all kinds of Internet pulley ‘calculators’ that you can use to find the various combinations of pulley sizes that you might need for your specific situation but in general the rule of thumb is one of ratios. For instance a motor fitted with a 2-inch pulley will drive a shaft fitted with a 4-inch pulley at one-half the motor speed. It’s a simple 2.0:1 ratio. The same motor driving a 5-inch pulley would have a 2.5:1 ratio and driving a 6-

inch pulley it would have a 3.0:1 ratio or speed reduction. It doesn’t take long to realize that a motor running at 1740rpm needs to be turning a shaft fitted with a 12-inch pulley in order to get the speed down to 290rpm. For a small table there probably won’t be enough room to use a 12-inch pulley so we need to look at multiple or staged speed reduction by using more than just two pulleys and pillow block bearing assemblies.

If you’re lucky enough to latch on to a motor that only turns at 1140 rpm you can get by with just a 2-inch primary drive pulley and an 8-inch final drive pulley which simplifies things tremendously.

All of this gear takes up a lot of space and in some cases the drive assembly will actually be larger in physical dimensions than your table.

Unfortunately I have no idea what kind of motive power system you’ll be using so I can’t give any hard dimensional drawings for this part of your particular table and this is one reason that I recommend that you build the motive system and it’s support frame as a separate element. You can bolt it to your table frame or incorporate it inside your carriage or stand when the table is finished.

I wish I could be of more help in this area but the variables are simply to great for me to illustrate specific design alternatives. As an example I set up two motors on the bench and both are rated as 1/4 horsepower. One is 120 volt and the other is 12 volt. Both motors have almost the same torque curves but as you can see in the snapshot the physical dimensions, shaft size and mounting points are radically different.

My recommendation is to design for accommodating an 8-inch pulley somewhere in your power transmission system, as this will be the largest pulley that you’ll probably ever use. Allowing room for this size then makes it easier to work things out if you developed a drive system that uses smaller pulleys.

A rudimentary schematic diagram of a simple power transmission system is shown in Figure 35.

Figure 35

This system would be appropriate if you have a variable speed motor or a conventional motor equipped with a speed controller or even a motor that ran at a fixed speed of around 300 rpm’s.

In action this system spins shaft ‘A’ that is mounted between a pair of pillow block bearing assemblies and in turn rotates ‘cam-3’ that actuates the eccentric control arm or hits the tables bumping block.

Figure 36 illustrates a more complicated system that relies on the use of various pulleys to reduce the speed of the final drive shaft.

Shaft ‘A’ is mounted between a pair of pillow blocks on the lower portion of the table support frame and shaft ‘B’ is mounted in a similar manner but onto the upper portion of the support frame. The entire assembly may be completely contained within the confines of the table frame or mounted on a platform at one end of the table frame.

In action the motor drives shaft ‘A’ at a fraction of the motor speed by use of a larger pulley. This reduced speed is then further reduced between shaft ‘A’ and shaft ‘B’ by an even larger pulley.

Figure 36

Shaft ‘B’ then drives the bumper cam or eccentric at around 300 rpm. This may look complicated but in application it is extremely simple and cost effective to implement compared to the cost of a motor speed controller; Note than I haven’t shown the belt tension idlers or other minor details in these simple illustrations.

Once you have bought or scrounged all of your drive gear you’ll be wondering how to cram 5 pounds of stuff into a 2-pound bag but it can be done with some creative design work.

Figure 37 depicts a portion of an engineering drawing for a small table equipped with a Gemini table type drive system and a rather large 115-volt motor. All of the gear fits into a space that is 24-inches long by 12-inches high and only 18-inches wide.

Figure 37

As compact as this particular installation is it can get even smaller if we leave behind the old 19th and 20th century design concepts on which it is based.

Most of the older table designs use what I call a ‘horizontal’ design scheme since they are easy and economical to build but newer tables, having very small footprints, are using a ‘vertical’ design scheme where the shafts are oriented vertically instead of horizontally. A good illustrative example of this scheme that most of us can relate to is the average modern drill press which can be adapted to table use quite easily.