The hammermill in most feed mills is hidden out of sight and often out of mind. Due to insurance companies traditionally wanting hammermills situated adjacent to an outside wall and invariably contained within its own secure environment with explosion relief directed toward the atmosphere, the hammermill is often squeezed into as small a space as possible.
With poor lighting and bad access, it tends to be an overlooked piece of feed milling equipment. Yet, it performs a vital function within the mill, one which many other machines rely upon. As a prime example, grist spectrum of the resultant ground material affects pelleted product quality and has a direct effect on power consumption at the press. In the case of poultry mashes, food conversion is often directly related to grist spectrum and particle size range. Without adequate and regular maintenance, the hammermill can have an adverse effect on several parameters and can cost the miller dearly if he is not aware of the potential pitfalls.
When looking at any capital expenditure, it is important to consider not only the initial capital outlay but also the likely reliability of the plant you are purchasing and the ease with which maintenance can be carried out. Hammermills are no exception. Your first consideration, however, must be whether you wish to adopt a pre-grind scenario or a postgrind scenario. In other words, do you wish to grind all raw materials separately prior to weighing and blending in the pre-grind scenario or afterward in the case of the post-grind scenario, where all raw materials are ground together?
Both scenarios have their relative merits, but the post-grind choice is more common since capital costs are reduced due to the reduced number of intermediate storage silos being adopted. The onus on the maintenance department is greater, however, as post grinding means that if the hammermill goes down for any reason, the entire mill is stopped. Additionally, any problems with burst screen or poor performance is immediately seen in the finished feed. If you have observant operatives, then this is not an issue.
But with automated press controls, which are common these days, it can sometimes be a while before problems are spotted. That’s why it’s very important to have good maintenance programs in place on a regular basis and, perhaps more importantly, to have regular inspections made of grist spectrum. The use of auto samplers can prove very effective in monitoring hammermill performance and is becoming more widely adopted in larger mills.
LUBRICATION IS KEY
What are the key issues of maintenance that should be addressed? Obviously lubrication is an important element of any maintenance program, and the choice of lubricant is key. All manufacturers have done their research and know what type of lubricant best serves their machines. They have a recommended schedule for greasing bearings and balance points, and they don’t just put schedules together for fun. Pay attention to their recommendations. Auto lubrication works fine on pelleting presses but does not work well on hammermills, mainly because there is a variable tonnage being processed. Getting the right quantity of lubricant applied is much better done by maintenance operatives, as they can judge whether a bearing is running hot or dry much better than any automated applicator.
Motor bearing lubrication must also be considered. Hammermills can generate a lot of heat, being frictional machines, and any out-of-balance condition will cause vibration in the bearings. Despite the quality of couplings used today, there is always the possibility that heat from the hammermill will be transmitted to motor bearings, causing lubricants to prematurely deteriorate and evaporate. Any program for lubrication should include drive motor bearings.
Choice of beater configuration is also something the manufacturer will have an opinion on, but the miller knows best and also understands the vagaries of the raw materials he is processing. Choosing the correct number and balanced configuration of beaters will assist hammermill performance and will enable power consumption to be optimized. Always use balanced sets of hammermill beaters. It may mean that you are taking off beaters a bit earlier than you would like, but you will not break as many, and power consumption will be reduced sufficiently enough to compensate for additional beater costs.
Screen sizes and type are crucial. An increase in screen perforation diameter will reduce power costs but also lead to an increased grist particle size. With auto screen changers ideally linked to recipe specifications in the blending control system, the miller can optimize his grists and maximize the efficiency of the hammermill. Keeping good screens available with sharp edges to the holes will pay dividends. One of the major problems with both beater and screen wear can be linked to incorrect and inappropriate feeding mechanisms. Narrow feeds that do not utilize the full width of the hammermill lead to irregular wear patterns, meaning that some beaters are unused and some areas of the screens are underutilized.
When carrying out inspections and maintenance work, the keen eye can tell when irregular feed is creating problems. In most cases, this can be resolved quickly and easily and is simply a matter of feeder mechanism adjustment. An inexperienced operative will often open up a feeder so there is less chance of it plugging up, not realizing that this allows the feed to run only through the centerline of the mill, not using the full width of screen and beaters available.
Turning beaters end for end, reversing rotation and mixing up screen sizes will all have a positive effect on maintenance costs and hammermill performance. But one of the key issues that is often neglected is airflow through the mill. When carrying out maintenance work, the hammermill is usually stopped, locked out and tagged, and the maintenance operative does not see and observe the air flow patterns through the mill. Good filter maintenance, observation of magnehelic manometers and regular attention to sleeves and fan drives will pay off when it comes to hammermill performance and power costs.
Poor airflow will lead to reduced product speed through the hammermill. Grains will dwell for longer periods of time in the hammermill grinding chamber, and the peripheral tip speed which you rely on to produce the fineness of grind will be nullified by product not being evacuated quickly enough through the screen. This not only leads to high power consumption and cost, but also to unnecessary wear on beaters and screens through product being retained when it should be on its way to the destination silo.
Even when hammermills are not discharged pneumatically, they need a considerable amount of air to assist with product flow through the screen. Some manufacturers have configured their machines in a horizontal plane, whereby the grinding rotor also acts as a fan and directs the airflow in the correct manner through the screen. These machines are ideal machines for mounting on the tops of mixers, where the mixer body then acts as an expansion chamber and the hammermill itself is neatly contained with good access for maintenance activities.
Access is an important part of any maintenance activity, particularly in the case of hammermills where screens and beaters need to be removed and refitted regularly. Additionally, kibbling plates or disruptor bars fitted in the top quadrant of a hammermill or at the junction of screens need to be examined regularly and replaced periodically. But it is often a case of changing annually rather than anything more frequently. Good light needs to be available for inspection when carrying out maintenance, and it is imperative when looking at grist spectrum to do so where variations can be seen clearly if you are not going to use a sieve to statistically measure particle size spread.
Good hammermill beater selection — using the right hardness with the correct and appropriate tip hardness — goes a long way to efficient hammermill performance. It is often tempting to buy cheaper beaters that are not through-hardened only to find that they perform well for a week or so and then shatter once the case hardness has been penetrated. Similarly, the choice of suitable metal for screens is paramount, and again the temptation to buy cheaply should be avoided unless you can be sure of screen quality. Remember that metallic fragments that disintegrate in a hammermill usually find their way into other machinery.
Putting magnets in front of every hammermill and check them regularly — at least once each shift —will tell you a lot about your mill, including which bits are in need of attention. Hopefully it will prevent damage to your hammermill and keep your mill operational and your customers and their livestock happy.
Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J B Bradshaw Ltd. He may be contacted at: firstname.lastname@example.org?.