The muller wheels in a muller type mixer are the heart of the mixer. The muller wheels provide the uniquely intensive kneading, shearing and spatulating actions that produce the excellent mixing and energy efficiency that muller type mixers are known for. To achieve optimal performance, the design criteria for the muller wheels in each model of the Simpson Mix-Muller and Simpson Multi-Mull has a very specific target for muller wheel dimensions and weight. These criteria have been developed over many years of research, development and operation in thousands of installations to provide the optimal combination of pressure (kneading) and shear across the width of the muller wheel face.
The muller wheels in a Simpson type muller mixer are vertically positioned in the mixer and rotate around a center axis. The wheels are positioned slightly off true center in order to increase the shearing action of the wheels. During operation the outside edge of the wheel furthest away from the mixer center will travel a greater distance than the inside edge of the wheel. This will result in accelerated wear along the outside edge of the wheel. A lack of maintenance could eventually result in a badly worn wheel that no longer has its original weight or a flat muller face to produce the effective shearing action of the original wheel. An example of a badly worn wheel that is no longer effectively mixing is shown in Figure 1. This wheel is well beyond its effective design specifications and would be totally ineffective in mixing.
To help maintain the performance of the mixer and extend the life of the muller wheel the G series muller wheels have always been reversible so that the tires can be rotated when wear occurs. However, it was frequently a judgement of the maintenance personnel when to rotate the tires and when the tire was fully worn it had to be discarded and replace. Some suppliers would take these worn wheels and coat them with urethane but the result was a wheel which did not meet the original, optimal design specifications for wheel weight and size which would reduce the mixing efficiency of the wheels and stress the performance of other components such as shafts, bearings, etc.
Why You Should Rotate Your Muller Wheels
Muller wheel rotation is important when it comes to maintaining the performance and safety of your mixer. Wheel rotation can be beneficial in several ways. When done at the recommended times, it can preserve the mixing performance of the wheel and extend the life of the wheel thereby reducing maintenance and operating costs. Proper rotation not only helps even out wear and extends the life of your wheels; it provides the perfect opportunity to make certain all wheel components are in good working order. It’s a good time to inspect the wheels for damage – particularly from heat or metal. It’s also a good time to check the condition of your wheel shafts, bearings, seals, etc. where you can’t easily see during normal maintenance.
Ideally, both wheels on a crosshead will wear evenly allowing them to perform equally well during mixing. Also, when all your wheels wear out together, you can replace them with an entire set of new wheels and the old set can be rebuilt or repaired in the convenience of a workshop rather than in the mixer. By replacing wheels in sets of two you’ll maintain the original handling balance of the tool set.
Development of the new urethane wheel for G series machines
In 2019 Simpson introduced a completely redesigned urethane muller wheel for the G series Simpson Mix-Mullers and Simpson Multi-Mulls. These new wheels are designed to provide exceptional performance in terms of wear and cleanliness and significantly reduce maintenance.
Each wheel has been designed to feature a cast iron core and a urethane outer layer that maintains the original wheel design weight and size in order to assure optimal mixing performance. Since urethane is less dense than cast iron, as the urethane wears less weight is lost when compared to traditional iron wheel. As the urethane wears the cast iron core will maintain the significant majority of the desired wheel weight.
To signal to the maintenance staff that it is time to rotate the wheel two sets of wear indicators are fastened to the cast iron core and embedded into high visibility yellow urethane. The yellow urethane is bright inside the muller and when the outer edge of the wheel wears the urethane down to the “high” indicator the maintenance staff will know it’s time to rotate the wheel. The process will repeat again and eventually the second “high” indicator will be revealed and the wheel will be rotated again. This process will repeat until both of the “low” indicators have been revealed and the wheel has gone through 3 rotations. When the last “low” indicator is revealed it is time to remove the wheel from the machine and replace it with a spare. The removed wheel can then be sent to Simpson Technologies where the worn urethane and wear indicators will be removed and replaced. The wheel can be repaired for a fraction of the cost of a totally new wheel.
As shown in Figure 3, lifting the wheel for rotation or removal has been dramatically simplified by placing four locations where a sling and lifting fixture can be attached through the wheel to securely hold the wheel to an overhead crane. All Simpson Mix-Mullers and Simpson Multi-Mulls also have removable mixer wall sections and dust hood sections that make it easy for maintenance personnel to enter the machine and remove large parts. Maintenance is safer and consumes less time for busy maintenance staff.
All of the mentioned improvements are available to upgrade any G series Simpson Mix-Muller or Simpson Multi-Mull and can be retrofitted to a mixer during a normal maintenance cycle.
A North American Iron Foundry upgraded to the new urethane coated muller wheels in their model 23G-250 Simpson Multi-Mull. By switching to the new wheels this customer achieved exceptional wheel wear life and reported more than 20,000 operational hours before the wheel was halfway through its life. At this rate, the new wheels are on target to run for 40,000 operational hours.
On average a one-shift operation is 2,000 hours per year, therefore the new urethane coated wheels would last for approximately 20 years. Likewise, a two-shift operation would average 10 years of wheel life and a three- shift operation would average more than 6 ½ years. This customer installed a crib wall scraper and urethane mixer wall liners to help keep the inside of their Multi-Mull clean and running at peak performance. The data recorded by this iron foundry supports our claim that the new design and the ability to easily rotate the wheels dramatically improves the life of these wear parts, improves their performance over their working life and lowers the cost of sand preparation.
For More Information
More information on this upgrade is available on New Urethane Coated Wheels Engineering Bulletin and can be viewed here.
You can either contact us directly from the engineering bulletin webpage or by emailing firstname.lastname@example.org for a quote on upgrades for your Simpson mixer. Please provide us with the model and serial number of your machine so that we can confirm the correct parts are specified for your upgrade. We can also provide field service assistance to guide your staff through the upgrade and train your team on how to install and maintain the new components.
*Results may vary depending on your process and application. *