Most of the 38 million pay, pension and benefit checks Uncle Sam sends citizens each month are now subject to withholding for alimony or child support.

Although many potential beneficiaries don't know it - or understand the workings of the law - there is legal machinery in place that requires the government to handle court orders for alimony and child support and take the money from the pay or pension check.

As many as 40,000 federal checks - mostly for pension or pay - are already being reduced to satisfy alimony and child-support claims before the employee, military man or retiree gets that check. In effect, the exspouse or local welfare agency (acting as a collector seeking reimbursement, for alimony and child-support funds paid out) get their legal cut off the top.

Most of the garnishment orders that have been handled so far are against government retirees, military personnel or active-duty civilian or military personnel paid by the federal government. The Civil Service Commission, for example, already is making alimony or child-support deductions from 400 federal checks, most of them pensions checks.

Army estimates it has about 6,000 garnishment actions going for alimony or child support. Federal officials say that other government units are also honoring alimony and child-support orders from courts. The U.S. Postal Service has processed a substantial number against employees and retirees. The claims can also be made against payments to consultants and D.C. government workers.

But the biggest potential source of alimony or child-support income remains virtually untapped. It is the Social Security system, which regularly issues 31 million checks monthly, mostly to the disabled or retired. Only about 100 of 31 million checks now are being garnished.

Federal officials expect a big jump in court orders for alimony and child-support, which the government will handle once individuals, the legal community and the courts fully understand that Social Security checks also are subject to garnishment. Up to 65 per cent of the amount in certain cases, can be garnished for alimony and child-support.

The procedures for garnisheeing a portion of a government paycheck or pension check are not particularly complicated. But they can be time-consuming, and while some legal aid groups, welfare offices and lawyers have been helpful on the matter, others have remained indifferent or ignorant of the procedures, helping reluctant federal agencies stall the badly needed payments.

Congress wrote the garnishment law - which was confusing and hard for agencies to administer - to get the burden of payments, particularly for child support, off taxpayers who pay for welfare, and get at the fathers (or mothers) who are not complying with court orders.

Now individuals with court orders can go directly to the paying agency of a former spouse, getting his or her check garnished even if the exspouse has disappeared. As long as he or she is getting a government check (with the exception of some VA payments) it can garnished at the source.

Most of the people eligible for alimony and child-support payments are women or mothers of dependent children. Many have tried to get pay or pensions checks garnished by calling their ex-husbands' offices. that isn't the way it works. It must go through the courts, and the court must issue an order for the payments, then that order is transmitted to the agency or military department responsible for paying the ex-spouse.

Generally people seeking the garnishment orders need a lawyer. Many lawyers handle the matter right and for a flat fee. But some have botched it - according to complaints to the government and this newspaper - or have demanded a permanent or longtime fee for their services based on a percentage of the monthly check. In some areas local welfare offices have tried to eliminate some of the court steps for clients by making the court-ordered payments directly to the spouse and then seeking reimbursement in the form of garnishment, from the federal agency.

At any rate, federal officials say the program of garnishment, which got off to a slow and confusing start, now is rolling, and they expect Uncle Sam soon will be in the middleman collection-agency role in a big way.